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diff --git a/old/50552-8.txt b/old/50552-8.txt deleted file mode 100644 index feb908b..0000000 --- a/old/50552-8.txt +++ /dev/null @@ -1,3996 +0,0 @@ -Project Gutenberg's Acids, Alkalis and Salts, by George Henry Joseph Adlam - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Acids, Alkalis and Salts - -Author: George Henry Joseph Adlam - -Release Date: November 26, 2015 [EBook #50552] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK ACIDS, ALKALIS AND SALTS *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net (This file was -produced from images generously made available by The -Internet Archive) - - - - - - - - COMMON COMMODITIES AND INDUSTRIES SERIES - - Each book in crown 8vo, cloth, with many illustrations, charts, etc., - 2/6 net - - - TEA. By A. Ibbetson - COFFEE. By B. B. Keable - SUGAR. By Geo. Martineau, C.B. - OILS. By C. Ainsworth Mitchell, B.A., F.I.C. - WHEAT. By Andrew Millar - RUBBER. By C. Beadle and H. P. Stevens, M.A., Ph.D., F.I.C. - IRON AND STEEL. By C. Hood - COPPER. By H. K. Picard - COAL. By Francis H. Wilson, M.Inst., M.E. - TIMBER. By W. Bullock - COTTON. By R. J. Peake - SILK. By Luther Hooper - WOOL. By J. A. Hunter - LINEN. By Alfred S. Moore - TOBACCO. By A. E. Tanner - LEATHER. By K. J. Adcock - KNITTED FABRICS. By J. Chamberlain and J. H. Quilter - CLAYS. By Alfred B. Searle - PAPER. By Harry A. Maddox - SOAP. By William A. Simmons, B.Sc. (Lond.), F.C.S. - THE MOTOR INDUSTRY. By Horace Wyatt, B.A. - GLASS AND GLASS MAKING. By Percival Marson - GUMS AND RESINS. By E. J. Parry, B.Sc., F.I.C., F.C.S. - THE BOOT AND SHOE INDUSTRY. By J. S. Harding - GAS AND GAS MAKING. By W. H. Y. Webber - FURNITURE. By H. E. Binstead - COAL TAR. By A. R. Warnes - PETROLEUM. By A. Lidgett - SALT. By A. F. Calvert - ZINC. By T. E. Lones, M.A., LL.D., B.Sc. - PHOTOGRAPHY. By Wm. Gamble - ASBESTOS. By A. Leonard Summers - SILVER. By Benjamin White - CARPETS. By Reginald S. Brinton - PAINTS AND VARNISHES. By A. S. Jennings - CORDAGE AND CORDAGE HEMP AND FIBRES. By T. Woodhouse and P. Kilgour - ACIDS AND ALKALIS. By G. H. J. Adlam - - - _OTHERS IN PREPARATION_ - - [Illustration: _Copyright by Messrs Flatters & Garnett, Manchester_ - BACTERIA NODULES ON THE ROOT OF LUPIN] - - PITMAN'S COMMON COMMODITIES AND INDUSTRIES - - - - - ACIDS, ALKALIS AND SALTS - - - BY - G. H. J. ADLAM, - M.A., B.Sc., F.C.S. - Editor of "The School Science Review" - - London - Sir Isaac Pitman & Sons, Ltd., 1 Amen Corner, E.C.4 - Bath, Melbourne and New York - - Printed by Sir Isaac Pitman & Sons, Ltd., London, Bath, Melbourne and - New York - - - - - PREFACE - - -It has often been said, and still more often implied, that -considerations of utility in education are incompatible with its main -object, which is the training of the mind. Extremely divergent views -have been expressed on this point. Schoolmen have looked askance at some -branches of knowledge because they were supposed to be tainted with the -possibility of usefulness in after life. On the other hand, business men -and others have complained bitterly of the present state of education -because very little that is considered "useful" has up to the present -been taught in schools. - -It is possible to err in both directions. A university professor, -lecturing on higher Mathematics, is reported to have told his audience -that it was a source of great satisfaction to him that the theorem which -he was demonstrating could never be applied to anything "useful." On the -other hand, we have the well-authenticated story of the man who took his -son to the Royal School of Mines to "learn copper," and not to waste his -time over other parts of Chemistry, because "they would be of no use to -him." - -For narrowness of outlook, there is nothing to choose between the pedant -and the "practical" man. National education would deteriorate if its -control should ever pass into the hands of extremists of either type, -for nothing worthy of the name of education could ever be given or -received in such an irrational spirit. - -In dealing with the subject of "Acids, Alkalis, and Salts," I have -endeavoured to give prominence to the commercial and domestic importance -of the substances dealt with. I thereby hope to gain the interest of the -reader, since interest stands in the same relation to education that -petrol does to the motor-car. It is not education itself, but it is the -source of its motive power. I have also included some considerations of -a theoretical nature which may well be taken as a first step towards the -continuation of the study of Chemistry. - -My sincere thanks are offered to my colleagues, F. W. G. Foat, M.A., -D.Litt., and Mr. I. S. Scarf, F.I.C., for much valuable help and advice; -to Sir Edward Thorpe, C.B., F.R.S., and Messrs. William Collins & Sons -for permission to reproduce Figures 3, 11, and 14; to Messrs. Longmans & -Co. for Figures 4, 5, 9, 12, 13, 16; Messrs. Macmillan & Co., for -Figures 8, 10 and 15. I have also availed myself of the assistance of -several standard works on Chemistry. My acknowledgments in this -direction take the practical form of the short bibliography which -follows-- - - - Lunge, Dr. G. - _The Manufacture of Sulphuric Acid and Alkali._ Vols. I, II, and - III. - Roscoe & Schorlemmer - _Treatise on Chemistry._ - Vol. I. The Non-metallic Elements (1911). - Vol. II. The Metals (1913). - Brannt, W. T. - _The Manufacture of Vinegar and Acetates._ - Thorp, F. H. - _Outlines of Industrial Chemistry_ (1913). - Thorpe, T. E. - _A Manual of Inorganic Chemistry._ - Newth, G. S. - _A Text-book of Inorganic Chemistry._ - Mellor, J. W. - _Modern Inorganic Chemistry._ - Cohen, J. B. - _Theoretical Organic Chemistry._ - - - G. H. J. A. - - - City of London School, E.C. - - - - - CONTENTS - - - CHAP. PAGE - PREFACE v - I. INTRODUCTION 1 - II. SULPHURIC ACID AND SULPHATES 10 - III. NITRIC ACID AND NITRATES 28 - IV. THE HALOGEN ACIDS 43 - V. CARBONIC ACID AND CARBONATES 49 - VI. PHOSPHORIC, BORIC, AND SILICIC ACIDS 56 - VII. ORGANIC ACIDS 67 - VIII. MILD ALKALI 80 - IX. CAUSTIC ALKALIS 95 - X. ELECTROLYTIC METHODS 101 - INDEX 109 - - - - - ILLUSTRATIONS - - - FIG. PAGE - BACTERIA NODULES ON THE ROOT OF LUPIN _Frontispiece_ - 1. DIAGRAM 7 - 2. PLAN OF SULPHURIC ACID WORKS 13 - 3. GENERAL VIEW OF SULPHURIC ACID WORKS 15 - 4. SULPHUR TRIOXIDE--THE CONTACT PROCESS 19 - 5. PREPARATION OF NITRIC ACID 30 - 6. NITROGEN CYCLE (DIAGRAM) 38 - 7. NITRIC ACID FROM AIR (DIAGRAM) 41 - 8. PREPARATION OF HYDROCHLORIC ACID 45 - 9. BORIC ACID 59 - 10. QUICK VINEGAR PROCESS 71 - 11. DUTCH PROCESS FOR WHITE LEAD 74 - 12. SALT CAKE FURNACE 83 - 13. BLACK ASH FURNACE 85 - 14. THE SOLVAY PROCESS 89 - 15. THE ELECTROLYSIS OF SALT SOLUTION 102 - 16. THE CASTNER PROCESS 105 - - - - - ACIDS, ALKALIS, AND SALTS - - - - - CHAPTER I - INTRODUCTION - - -Acids. A vague hint from Nature gave mankind the first indication of the -existence of acids. The juice pressed from ripe grapes is a sweetish -liquid. If it is kept for some time, the sweetness goes, and the liquid -acquires a burning taste. If kept still longer, the burning taste is -lost, and in its place a sharp acid flavour, not entirely displeasing to -the palate, is developed. The liquid obtained in this way is now called -wine vinegar; the particular substance which gives it its characteristic -taste is acetic acid. - -The strongest vinegar does not contain more than 10 per cent. of acetic -acid, which is itself a comparatively weak acid. It is, therefore, not a -very active solvent. Nevertheless, for metals and for limestone rock, -and other substances of a calcareous nature, its solvent power is -greater than that of any other liquid known at the time of its -discovery. It was this property which seems to have appealed most -strongly to the imagination of the early chemists; and, as is very often -the case, the description of its powers was very much exaggerated. Livy -and Plutarch, who have given us an account of Hannibal's invasion of -Italy by way of the Alps, both gravely declare that the Carthaginian -leader cleared a passage for his elephants through solid rocks by -pouring vinegar over them! - -In the Middle Ages, the study of Chemistry was fostered mainly as a -possible means whereby long life and untold riches might be obtained. -The "Philosopher's Stone," by the agency of which the base metals were -to be changed to gold, and the "Elixir of Life," which was to banish -disease and death, were eagerly sought for. Though these were vain -imaginings according to modern ideas, nevertheless they were powerful -incentives towards experimental work. Many new substances were -discovered in this period, and among these were nitric acid (aqua -fortis), hydrochloric acid (spirit of salt), and sulphuric acid (oil of -vitriol). - -Acids were then valued above all other substances. The mediaeval chemist -(or alchemist, as he was called) clearly saw that unless a body could be -dissolved up there was no hope of changing it. Nitric acid, therefore, -which, in conjunction with hydrochloric acid, dissolved even gold -itself, was very highly esteemed. Oil of vitriol also was scarcely less -important, for it was required for the production of other acids. - -So far, taste and solvent power were considered to be the characteristic -feature of acids. In the time of Robert Boyle (1627-1691), they were -further distinguished from other substances by the change which they -produced in the colour of certain vegetable extracts. Tincture of red -cabbage was first used, but, as this liquid rapidly deteriorates on -keeping, it was soon replaced by a solution of litmus, a colouring -matter obtained from _Roccella tinctoria_ and other lichens. It imparts -to water a purple colour, which is changed to red by the addition of -acids. - -Alkalis. Wood ashes were valued in very early times because they were -found to be good for removing dirt from the skin. Mixed with vegetable -oil or animal fat, they formed a very primitive kind of soap, which was -afterwards much improved by using the aqueous extract instead of the -ashes themselves, and also by the addition of a little caustic lime. - -When plant ashes are treated with water, about 10 per cent. dissolves. -If the insoluble matter is then allowed to settle down and the clear -liquid evaporated to dryness, a whitish residue is obtained. The soluble -matter thus extracted from the ashes of plants which grow in or near the -sea is mainly soda; that from land plants, mainly potash. Formerly no -distinction was made, and the general term "alkali" was applied to both. - -In order to bring the properties of alkalis into contrast with those of -acids, we cannot do better than make a few simple experiments with a -weak solution of washing soda. Its taste is very different from that of -an acid; it is generally described as caustic. If a little is rubbed -between the fingers, it feels smooth, almost like very thin oil. It does -not dissolve metals or limestone. Its action on vegetable colouring -matter is just as striking as that of acids. Tincture of red cabbage -becomes green; the purple of litmus is changed to a light blue. This -colour change is characteristic of alkalis. - -Neutralization. When the colour of litmus solution has been changed to -red by the addition of an acid, the original colour can be restored by -adding an alkali. The change can be repeated as often as desired by -adding acid and alkali alternately. From this we get a distinct -impression of antithesis between the two. In popular language, an alkali -"kills" an acid; in Chemistry, the same idea is expressed by the term -"neutralization." - -Salts. Both "neutralization" and "killing the acid" are modes of -expression which describe the phenomenon fairly well. When an acid is -neutralized, its characteristic taste, its solvent power, and its action -on litmus, are all changed; in fact, the acid as an acid ceases to -exist, and so does the alkali. When the neutral solution is evaporated -to dryness, a residue is found which on examination proves to be neither -the acid nor the alkali, but a compound formed from the two. This -substance is called a salt. - -To most people, salt is the name for that particular substance which is -taken as a condiment with food. Its use in this connection dates from -time immemorial. It is distinctly unfortunate that another and very much -wider usage of the term has been introduced into Chemistry. When the -early chemists recognized that other substances, which they vaguely -designated as "saline bodies," were similar to common salt in -composition, they took the name of the individual and applied it to the -whole class. - - - OTHER METHODS OF SALT FORMATION - -Solution of Metals in Acids. Alkalis are not the only substances which -neutralize acids. Speaking in a broad and general sense, we may say that -an acid is neutralized when a metal is dissolved in it, because, when -the point is reached at which no more metal will dissolve, all the -characteristic properties of the acid are destroyed. A salt is formed in -this case also. - -An example will now be given to illustrate this method of salt -formation. Before two pieces of metal can be united by soldering, it is -necessary to clean the surfaces of the metal and the soldering iron. The -liquid used for this purpose is made by adding scraps of zinc to -muriatic acid (hydrochloric acid). The zinc dissolves with -effervescence, which is caused by the escape of hydrogen gas. When -effervescence ceases and no more zinc will dissolve, the liquid is ready -for use. The acid has been "killed" or neutralized by the metal. A salt -called zinc chloride has been formed. This salt can be recovered from -the liquid by evaporation. - -Solution of Oxides in Acids. The substances most used in commerce with -the express purpose of destroying acidity are quicklime, washing soda, -and powdered chalk. - -Since quicklime is a compound of the metal calcium and the gas oxygen, -its systematic name is calcium oxide; when it neutralizes an acid, it -forms the corresponding calcium salt; for example, if it neutralizes -acetic acid, calcium acetate is formed. - -An instance of the neutralization of an acid by an oxide of a metal is -furnished by one method of preparing blue vitriol (copper sulphate). -Copper does not dissolve very quickly in dilute sulphuric acid; hence, -to make blue vitriol from scrap copper, the metal is first heated very -strongly while freely exposed to air. Copper and oxygen of the air -combine to form the brownish black powder, copper oxide, and this -dissolves very readily in sulphuric acid, making the salt, copper -sulphate. - -Solution of Carbonates in Acids. Washing soda and chalk belong to a -different class of chemical substances. They are carbonates, that is, -they are salts of carbonic acid. At first it may seem a little -perplexing to the reader to learn that a salt can neutralize an acid to -form a salt. It must be remembered, however, that acids differ from one -another in strength, that is, in chemical activity, and that carbonic -acid is a weak acid. When a salt of carbonic acid--sodium carbonate or -washing soda, for example--is added to a stronger acid such as sulphuric -acid, sodium sulphate is formed and carbon dioxide liberated. - -As an example of the neutralization of acids by carbonates, we may -mention here a practical sugar saving device. Unripe fruit is very sour -because it contains certain vegetable acids dissolved in the juice. -These acids are not affected by boiling; and, therefore, to make a dish -of stewed fruit palatable, it is necessary to add sugar in quantity -sufficient to mask the sour taste. If a pinch of bicarbonate of soda is -added to neutralize the acid, far less sugar will be necessary for -sweetening. - -Insoluble Salts. The methods given above apply only to those salts which -are soluble in water. Insoluble salts are obtained by mixing two -solutions, the one containing a soluble salt of the metal, and the -other, a soluble salt of the acid or the acid itself. - -The formation of an insoluble salt by the interaction of two soluble -substances is well illustrated in the preparation of Burgundy mixture, -the most effectual remedy yet proposed for checking the spread of potato -disease. This mixture contains copper carbonate, that is, the copper -salt of carbonic acid. For its preparation we require copper sulphate -and sodium carbonate (washing soda), a soluble carbonate. When these two -substances, dissolved in separate portions of water, are mixed, copper -carbonate is formed as a pale blue solid which is in such a state of -fine subdivision that it remains suspended in the solution of sodium -sulphate, the other product of the reaction. - -The change is represented diagrammatically below. Each circle represents -the atom or a group of atoms named therein. At the moment of mixing, -these groups undergo re-arrangement. - -Bordeaux mixture, which some gardeners prefer, is a similar preparation -containing copper hydroxide instead of copper carbonate. It is made by -mixing clear lime water (a soluble hydroxide) with copper sulphate. - - [Illustration: Fig. 1] - -Elements and Compounds. It is scarcely possible to discuss chemical -processes without having from time to time to use terms which are not in -everyday use. A few preliminary definitions and explanations of terms -which will be frequently used may serve to simplify descriptions, and -render it unnecessary to encumber them with purely explanatory matter. - -Among the many different kinds of materials known, which in the -aggregate amount to several hundreds of thousands, there are about -ninety substances which up to the present time have not been broken up -into simpler kinds. These primary materials are called "elements," the -remainder being known as "compounds." - -The following is a list of the commonest of these elements, together -with the symbols by which they are represented in Chemistry. - - METALS - Aluminium Al. - Antimony (_Stibium_) Sb. - Barium Ba. - Bismuth Bi. - Cadmium Cd. - Calcium Ca. - Chromium Cr. - Copper (_Cuprum_) Cu. - Gold (_Aurum_) Au. - Iron (_Ferrum_) Fe. - Lead (_Plumbum_) Pb. - Lithium Li. - Magnesium Mg. - Manganese Mn. - Mercury (_Hydrargyrum_) Hg. - Nickel Ni. - Platinum Pt. - Potassium (_Kalium_) K. - Silver (_Argentum_) Ag. - Sodium (_Natrium_) Na. - Strontium Sr. - Tin (_Stannum_) Sn. - Zinc Zn. - - NON-METALS - Boron B. - Bromine Br. - Carbon C. - Chlorine Cl. - Fluorine F. - Hydrogen H. - Iodine I. - Nitrogen N. - Oxygen O. - Phosphorus P. - Silicon Si. - Sulphur S. - -The first step in the building-up process consists of the union of a -metallic with a non-metallic element. Such compounds are binary -compounds, and are distinguished by the termination -ide added to the -name of the non-metallic element; for example, copper and oxygen unite -to form copper oxide, sodium and chlorine form sodium chloride, iron and -sulphur form iron sulphide or sulphide of iron. - -A compound containing more than two elements is distinguished by the -termination -ate. Most salts fall within this category; thus we speak of -acetate of lead and chlorate of potash, also of sodium sulphate and -copper sulphate, the latter form being the more correct. - -A difficulty arises when two bodies are composed of the same elements -combined in different proportions. Then we have to resort to other -distinguishing prefixes or suffixes. For this reason we meet with -sulphur_ous_ acid and sulphur_ic_ acid, the corresponding salts being -sulph_ites_ and sulph_ates_. - -Crystals and Water of Crystallization. When a soluble salt is to be -recovered from its solution, the latter is reduced in bulk by -evaporation until, either by experience or by trial, it becomes evident -that the solid will be formed as the liquid cools. In some cases, when -time is not an important factor, evaporation is left to take place -naturally. Under either set of conditions, the substance generally -separates out in particles which have a definite geometrical form. These -are spoken of as crystals. - -Crystals often contain a definite percentage of water, called "water of -crystallization." In washing soda, this combined water forms nearly 63 -per cent. of the total weight; in blue vitriol, it is approximately 36 -per cent. On being heated to a moderate temperature, the water is -expelled from the solid; the substance which is left behind is called -the anhydrous (that is, the waterless) salt. - - - - - CHAPTER II - SULPHURIC ACID AND SULPHATES - - -Key Industries. The importance of the chemical industries depends mainly -on the fact that they constitute the first step in a series of -operations by which natural products are adapted to our needs. The -materials which are found in earth, air, and water are both varied in -kind and abundant in quantity, but in their natural state they are not -generally available for immediate use. Moreover, very many substances -now deemed indispensable are not found ready formed in Nature. - -The end product of the chemical manufacturer is often one of the primary -materials of some other industry. Soda ash and Glauber's salt are -essential for making glass; soap could not be produced without caustic -alkali; the textile trade would be seriously handicapped if bleaching -materials, mordants, and dye-stuffs were not forthcoming. Considered in -this light, the preparation of chemicals is spoken of as a "key -industry." - -Furthermore, very few of these indispensable substances can be made -without using sulphuric acid. This acid is, on that account, just as -important to chemical industries as the products of these are to other -branches of trade. It may, therefore, be looked upon as a master key of -industrial life. - -Primary Materials. The composition of sulphuric acid is not difficult to -understand. Air is mainly a mixture of oxygen and nitrogen; and when a -combustible body burns, it is because chemical action between the -material and oxygen is taking place. In this way, sulphur burns to -sulphur dioxide. This gas, dissolved in water, forms sulphur_ous_ acid, -which changes slowly to sulphur_ic_ acid by combination with more -oxygen. Hence, sulphur, oxygen, and water are the primary materials -required for making sulphuric acid. - -Sulphur is the familiar yellow solid commonly known as brimstone. It is -found native in the earth, and is fairly abundant in certain localities, -notably in the neighbourhood of active and extinct volcanoes. Italy, -Sicily, Japan, Iceland, and parts of the United States are the principal -sulphur-producing countries. Though very plentiful and consequently -cheap, only a relatively small quantity of sulphuric acid is made -directly from native sulphur, because at the time when this industry was -started in England, restrictions were placed on the export of sulphur -from Sicily and, consequently, the plant which was then established was -adapted to the use of iron pyrites. - -Iron pyrites contains about 53 per cent. of sulphur combined with 47 per -cent. of iron, and when this is burnt in a good draught, nearly the -whole of the sulphur burns to sulphur dioxide, leaving a residue of -oxide of iron which can be used for making cast iron of a low grade. - -Iron pyrites is often supplemented by the "spent oxide" from the gas -works. Crude coal gas contains sulphur compounds which, if not removed, -would burn with the gas and form sulphur dioxide. The production of -these pungent and suffocating fumes would be a source of great -annoyance, and therefore it is necessary to remove the sulphur -compounds. To do this, the gas is passed through two purifiers, the -first containing slaked lime and the second ferric oxide, both in a -slightly moist condition. After being some time in use, the purifying -material loses its efficacy; the residue from the lime purifier is sold -as "gas lime," but that from the ferric oxide purifier is exposed to the -air and so "revived." At length, however, it becomes so charged with -sulphur that it is of no further use for its original work. It is then -passed on to the sulphuric acid maker. - -Evolution of the Manufacturing Process. In dealing with the main -processes for the manufacture of acids and alkalis, reference will -frequently be made to the methods of bygone times. Although as an exact -science Chemistry is comparatively modern, as a branch of human -knowledge its history goes back to the dawn of intelligence in man. It -is agreed that the higher types of living things are more easily -understood when those of a simpler and more primitive character have -been studied. In like manner, the highly specialized industries of -modern times become more intelligible in the light of the efforts of -past generations to achieve the same object. - -Basil Valentine, who lived in the fifteenth century, states that the -liquid which we now call sulphuric acid was in his day obtained by -heating a mixture of green vitriol and pebbles. Until quite recent -times, sulphuric acid of a special grade was made by precisely the same -method, except that the pebbles were dispensed with. In passing, we may -remark that the common name "vitriol," or "oil of vitriol," is accounted -for by this connection with green vitriol. The second method, quoted by -Basil Valentine, consisted of the ignition of a mixture of saltpetre and -sulphur in the presence of water. This is actually the modern lead -chamber process in embryo. - - [Illustration: Fig. 2. PLAN OF SULPHURIC ACID WORKS] - -About the middle of the eighteenth century, "Dr." Ward took out a patent -for the manufacture of sulphuric acid, to be carried on at Richmond in -Surrey. He used large glass bell jars of about 40-50 galls. capacity, in -which he placed a little water and a flat stone to support a red-hot -iron ladle. A mixture of saltpetre and sulphur was thrown into the ladle -and the mouth of the vessel quickly closed. After the vigorous chemical -action was over, the ladle was re-heated and the process repeated until -at last fairly concentrated sulphuric acid was produced. - -The large glass vessels used by Ward were costly and easily broken. They -were soon replaced by chambers about 6 ft. square, made of sheet lead, -but otherwise the process was just the same. The next advance consisted -in making the process continuous instead of intermittent. An enormously -increased output was thereby rendered possible, and the main features of -the modern process gradually developed. - -The Lead Chamber Process. We can now consider the actual working of the -lead chamber process, aided by the diagrammatic plan of the works shown -in Fig. 2. Sulphur dioxide is produced in a row of kilns (A-A) by -burning iron pyrites in a carefully regulated current of air. The -mixture of gases which leaves the pyrites burners contains sulphur -dioxide, excess of oxygen, and a very large quantity of nitrogen. To -this is added the vapour of nitric acid, generated from sodium nitrate -and concentrated sulphuric acid contained in the "nitre pots," which are -placed at B. The mixture of gases then passes up the Glover tower (C) -and through the three chambers in succession, into the first two of -which steam is also introduced. Sulphuric acid is actually produced in -the chambers, and collects on the floors, from which it is drawn off -from time to time. The residual gas from the last chamber is passed up -the Gay Lussac tower (D), and after that is discharged into the air by -way of the tall chimney (J). - - [Illustration: Fig. 3. GENERAL VIEW OF SULPHURIC ACID WORKS] - -The Oxygen Carrier. We have seen that sulphur dioxide, oxygen, and water -are the only substances required to produce sulphuric acid. Why, then, -is the nitric acid vapour added to the mixture? As described in a former -paragraph, the combining of these gases was represented as being a very -simple operation. So indeed it is, for it even takes place -spontaneously. Yet, as a commercial process, it would be quite -impracticable without the nitric acid vapour, for although the gases -combine spontaneously, they do so very slowly, and it is the nitric acid -vapour which accelerates the rate of combination. - -It is not known with any degree of certainty how the nitric acid acts in -bringing about this remarkable change. It has been suggested that -reduction to nitrogen peroxide first takes place, and that sulphur -dioxide takes oxygen from this body, reducing it still further to nitric -oxide, which at once combines with the free oxygen present to form -nitrogen peroxide again. So the cycle of changes goes on, the nitrogen -peroxide playing the part of oxygen carrier to the sulphur dioxide; and -since it is continually regenerated, it remains at the end mixed with -the residual gases. - -Recovery of the Nitrogen Peroxide. If the gases from the last chamber -passed directly into the chimney shaft, there would be a total loss of -the oxides of nitrogen, and the consequence of this would be that more -than 2 cwt. of nitre would have to be used for the production of 1 ton -of sulphuric acid. This would be a serious item in the cost of -production, and it is therefore essential that this loss should be -prevented. - -The recovery of the oxides of nitrogen is effected in the Gay Lussac -tower, a structure about 50 ft. in height, built of sheet lead and lined -with acid-resisting brick. It is filled with flints, over which a slow -stream of cold concentrated sulphuric acid is delivered from a tank at -the top. As the gas from the last chamber passes up this tower, it meets -the stream of acid coming down. This dissolves and retains the nitrogen -peroxide. The acid which collects at the bottom of the tower is known as -nitrated vitriol. - -The next step is to bring the recovered nitrogen peroxide again into -circulation. The nitrated vitriol is raised by compressed air to the top -of the Glover tower, and as it trickles down over the flints in this -tower it is diluted with water, while at the same time it meets the hot -gases coming from the pyrites burners. Under these conditions, the -nitrogen peroxide is liberated and carried along by the current of gas -into the first lead chamber. The stream of cold acid coming down the -Glover tower also serves to cool the hot gases before they enter the -first chamber. - -In order to complete the description of the works, it is necessary to -add a note on the lead chambers themselves. The sheet lead used in their -construction is of a very substantial character; it weighs about 7 lb. -per square foot. The separate strips are joined together by autogenous -soldering, that is, by fusing the edges together. In this way the -presence of another metal is avoided; otherwise this would form a -voltaic couple with the lead, and rapid corrosion would take place. - -The size of the chambers has varied a great deal. In the early years of -the nineteenth century, the capacity of a single chamber was probably -not more than 1,000 cu. ft.; at the present time, 38,000 cu. ft. is an -average size, and there may be three or five of these chambers. The -necessity for this large amount of cubic space is easily accounted for. -The reaction materials are all gases, and a gas occupies more than one -thousand times as much space as an equal weight of a solid or liquid. -Moreover, oxygen constitutes only about one-fifth of the total volume of -air used in burning the pyrites; the other four-fifths is mainly -nitrogen, which, though it does not enter into the reaction at all, has -to pass through the chambers. - -Modern Improvements. Among the modern innovations in the lead chamber -process, the following are worthy of note. "Atomized water," that is, -water under high pressure delivered from a fine jet against a metal -plate, has certain advantages over steam. In order to bring about a more -rapid mixing of the gases in the chamber, it is proposed to make these -circular instead of rectangular, and to deliver the gases tangentially -to the sides. Another suggestion is to replace the lead chambers by -towers containing perforated stoneware plates set horizontally. By this -arrangement, since the holes are not placed opposite one another, the -gases passing up the tower must take a zig-zag course. This makes for -more efficient mixing. - - - THE CONTACT PROCESS - -Sulphur Trioxide. When elements are combined in different proportions by -weight, they produce different compounds. Thus, in the case of sulphur -and oxygen, there are two well-known compounds, namely, sulphur dioxide -and sulphur trioxide. In the former, a given weight of oxygen is -combined with an _equal_ weight of sulphur; in the latter, this same -weight of sulphur is combined with 50 per cent. more oxygen. On this -account, sulphur trioxide is spoken of as the higher oxide. - -We can now state in general terms another method by which sulphuric acid -can be built up from its elements. Sulphur, as we have seen, burns in -oxygen, forming sulphur dioxide. This substance can then be made to -unite with more oxygen to give sulphur trioxide, which, with water, -yields sulphuric acid. There are three steps in this synthesis. The -first, namely, sulphur to sulphur dioxide, has already been considered; -the last, sulphur trioxide to sulphuric acid, only requires that sulphur -trioxide and water shall be brought together: we can, therefore, confine -our attention to the intermediate step, namely, the conversion of -sulphur dioxide into trioxide. - -This operation, when carried out in a chemical laboratory, is a very -simple one. Fig. 4 shows the necessary apparatus. Sulphur dioxide from a -siphon of the liquefied gas and air from a gasholder are passed into the -Woulff's bottle A, containing concentrated sulphuric acid; this removes -moisture from the gases. The drying process is completed in the tower B, -which contains pumice stone soaked in sulphuric acid. The mixed gases -then pass through the tube C, containing platinized asbestos heated to -about 400° C.: the sulphur trioxide collects in the cooled receiver D. - - [Illustration: Fig. 4. SULPHUR TRIOXIDE--THE CONTACT PROCESS] - -Platinized asbestos is made by soaking long-fibred asbestos in a -solution of platinum chloride. The material is then dried and subjected -to a gentle heat. In this way, metallic platinum in an exceedingly fine -state of subdivision is deposited on the asbestos fibre, which merely -serves as a convenient support. - -Catalytic or Contact Action. The influence of the finely divided -platinum is a very important factor in the reaction. It cannot, however, -be said to _cause_ the union of sulphur dioxide with oxygen, for the -gases combine to a very slight extent when it is not present. What the -platinum actually does is to influence the rate of formation to such a -degree that, under favourable conditions, practically the whole of the -sulphur dioxide is changed to sulphur trioxide instead of an exceedingly -small fraction of it. - -The most interesting, and at the same time the most perplexing, feature -of the reaction is that the platinum itself does not appear to undergo -any change. It is not diminished in quantity, for only a very small -amount is necessary for the conversion of a very large amount of the -mixed gases. Its activity lasts for a very long time, and even when it -does become inactive, it can be shown that this is due to some external -cause, such as the presence of dust and certain impurities in the gases. - -Many other similar cases are known in which the presence of a small -quantity of a third substance greatly influences the course of a -chemical reaction without appearing in any other way to be necessary to -the reaction. These substances, which are often metals in a very fine -state of subdivision, are called catalytic or contact agents. - -The Contact Process for making sulphuric acid is nothing more nor less -than the simple laboratory operation which we have described above, -carried out on a larger scale. - -The sulphur dioxide is produced as in the lead chamber process by -roasting iron pyrites in a current of air. This gas, together with the -excess of air, is passed into the contact furnace, which consists of -four tubes, each containing platinized asbestos, supported on perforated -plates. The union of the two gases is said to be almost complete: an -efficiency of 98 per cent. of the theoretical value is claimed for this -process. The sulphur trioxide, or "sulphuric anhydride"[1] is either -condensed in tin-lined drums or absorbed in ordinary concentrated -sulphuric acid. - -The proposal to manufacture sulphuric acid by this method was first made -in 1831 by Peregrine Phillips, of Bristol. The early attempts were not -successful, and it was not until about forty-four years later that the -difficulties arising in the working of the contact process were overcome -sufficiently to enable the sulphuric acid produced in this way to be -sold at the same price as that made by the lead chamber process. Since -1890, the total quantity of acid made by the contact method has -increased very rapidly, so that it now furnishes about one-half of the -world's supply, and seems likely in time to displace the lead chamber -process altogether. - -The history of the rise of the contact process is interesting because it -illustrates in a striking manner the very great difference that there is -between a successful laboratory process and a successful manufacturing -process, though seemingly identical. - -The first and possibly the most serious difficulty encountered in the -working of the contact process was the frequent interruption caused by -the loss of activity of the contact substance. Iron pyrites always -contains arsenic which volatilizes on heating, and this quickly caused -the platinum to lose its activity, or, as it was sometimes rather -fancifully expressed, "poisoned the catalyst." Dust also is inevitable, -and this, carried forward mechanically with the stream of gas, settled -on the contact substance and caused the action to cease. - -To get over this difficulty it is necessary to purify the gases. They -are first passed slowly through channels in which the coarser particles -of dust settle down. Steam is injected into the mixture to wash out the -finer particles of solid, and also to get rid of arsenic, and then the -gases are passed through scrubbers. Before being admitted to the contact -furnace, the moist gas is submitted to an optical test. It is passed -through a tube, the ends of which are transparent; a bright light is -placed at one end and viewed from the other through a column of gas of -considerable length. If the purification process is working -satisfactorily, there is a complete absence of fog. The gases are then -dried by passing through concentrated sulphuric acid and admitted to the -contact tubes. - -In all operations carried out on a large scale, the regulation of -temperature is a matter of some difficulty. In the case which we are -considering, the most suitable temperature range is a rather narrow one, -and the difficulty of keeping within the limits is very much increased -by the large amount of heat given out when the sulphur dioxide and -oxygen combine. The result of the failure to maintain the temperature at -a fairly constant level was that the process worked in a very irregular -manner, for as soon as it was working really well and sulphur trioxide -was being formed rapidly, the heat given out by the reaction itself was -also great, and consequently, the higher temperature limit was exceeded. - -The method of controlling the temperature in the contact process is -worth noting, because it is really ingenious. The tubes containing the -platinized asbestos are surrounded by wider concentric tubes. The gases -which are about to enter the contact furnace pass through the annular -space between the two tubes, and are thereby heated to the required -temperature, while at the same time they serve to cool the inner tubes. -The most satisfactory temperature is about 400° C. The tubes are first -warmed to 300° C. to start the reaction, and thereafter the heat evolved -by the reaction itself is sufficient to keep it going. - -The absorption of the sulphur trioxide also caused some difficulty at -first. This substance reacts most violently with water, dissolving with -a hissing sound like that produced when a red-hot poker is plunged into -water. At the same time great heat is developed, and consequently, much -of the sulphur trioxide is vaporized, and in that way lost. This -difficulty was got over by using 98 per cent. sulphuric acid for the -absorption, the acid being kept at this strength by the simultaneous -addition of water. - -The contact process has some very distinct advantages over the older -lead chamber process. The plant covers a much smaller area than the -bulky lead chambers. Although the preliminary purification of the gases -is somewhat tedious and costly, this is in great measure compensated by -the purity of the acid produced. No separate plant is required for -concentration and purification, as in the older process. Finally, -sulphuric acid of any concentration can be produced at will, including -the fuming acid, which is required as a solvent for indigo, and in the -manufacture of artificial indigo and other organic chemicals. - -The lead chamber process produces what is called chamber sulphuric acid -very cheaply. Although this is only a 60-70 per cent. solution and very -impure, nevertheless, it is quite good enough for the heavy chemical -trade, particularly for the first stage of the Leblanc soda process, and -for making superphosphate. These two industries alone consume many -thousands of tons of this sulphuric acid every year. Probably for some -years to come the two processes will continue to exist side by side, but -it may be doubted whether new works will now be installed to make -sulphuric acid by the lead chamber process. - -Properties of Sulphuric Acid. The pure non-fuming acid is a colourless -oily liquid whose density is 1·84. It mixes with water in all -proportions, yielding dilute sulphuric acid, and it also dissolves -sulphur trioxide, yielding the fuming acid. - -The mixing of sulphuric acid and water is accompanied by an evolution of -heat and by contraction in volume. It is an operation which must be -carried out with great care, the acid being always poured into the -water, otherwise the water floats on the heavier acid, and so much heat -is developed at the surface of separation that some of the water will be -suddenly converted into steam, and this, escaping from the liquid with -explosive violence, may cause the contents of the vessel to be scattered -about. - -Strong sulphuric acid chars most organic substances. From substances -such as wood, sugar, paper, starch, it withdraws the elements of water, -liberating carbon. Since it acts in the same way upon human flesh, it is -clear that the concentrated acid must be handled with very great care, -for it causes most painful burns. For this reason, vitriol throwing has -always been regarded as a most serious and dastardly offence. A simple -first-aid remedy for burns produced by sulphuric acid is the liberal -application of an emulsion of linseed oil and lime water. The lime, -being an alkali, neutralizes the acid, and the oil excludes air from the -wound. - -The readiness with which sulphuric acid combines with water is often -made use of both in the laboratory and in industrial Chemistry for the -purpose of drying gases. One illustration of this use has already been -given in describing the contact process. Another instance which may be -fairly familiar occurs in the case of liquefying air, where the gas must -be thoroughly dried before being passed into the refrigerating -apparatus, otherwise this would soon become blocked with ice. - -The position which sulphuric acid occupies in Chemistry is due mainly to -three outstanding features. In the first place, it is a strong mineral -acid and displaces all other acids from their salts. Secondly, it has a -high boiling point (338° C.), and consequently, the displaced acid with -the lower boiling point can be distilled from the mixture. Lastly, -sulphuric acid can be made very cheaply from materials which are very -abundant in Nature, and, therefore, it meets all the requirements of an -acid which is to be used for general purposes. - - - SULPHATES - -All the common metals, except gold and platinum, dissolve either in -concentrated or in dilute sulphuric acid, forming sulphates. These salts -are highly important and interesting substances. They are all soluble in -water, with the exception of the sulphates of calcium, strontium, -barium, and lead. - -Ferrous Sulphate, also called green vitriol and copperas, is obtained by -dissolving iron in dilute sulphuric acid. The solution is green, and -when it is evaporated, the crystals which separate out look like bits of -green glass. It was because of this that the substance was first called -green vitriol (_vitrum_ = glass). It is used very largely in dyeing as a -mordant. Writing ink and Prussian blue are also made from it. - -The Alums are double sulphates. They are made by crystallizing solutions -of potassium, sodium, or ammonium sulphate together with solutions of -iron (ferric), chromium, or aluminium sulphates. In this way, we may -have potassium aluminium alum, or iron ammonium alum, and so on, but -whichever combination of elements is present, the salt which is formed -always crystallizes in octahedra. The chief use of the alums, as also of -aluminium sulphate, is as mordants in dyeing. - -Since a great many metallic salts, particularly acetates and sulphates, -are used in the dye industry as mordants, it may be well to explain here -very briefly what a mordant is. - -It must be remembered that almost all the dyes are solids which dissolve -in water, yielding intensely coloured solutions. Hence, in most cases, -if a fabric is merely dipped in the dye and then dried, the colouring is -not permanent, but can be washed out with water. In order to fix the -colouring matter, the material is first dipped in the mordant, usually a -bath of some metallic salt, and then, generally after exposure to air or -after steaming, into the dye bath, with the result that the colour -becomes fixed. The first part of the process is called "mordanting" the -material. The mordant either adheres to or combines with the fibres, and -the dye forms with the mordant a coloured compound called a "lake," -which resists the action of water. The colour is then said to be "fast," -that is, firmly fixed. - -For printing on calico, the mordant is thickened with gum arabic or -other glutinous substance. The design is then stamped on the material -with the thickened mordant liquor. The subsequent treatment consists of -dipping the material in the dye and afterwards in water, when the colour -comes away from those parts which have not received the impress of the -mordant. - -Sodium Sulphate, or Glauber's salt, is made from common salt by the -action of concentrated sulphuric acid. It is one of the raw materials -used in making glass. - -Ammonium Sulphate. (_See_ p. 99.) - -Calcium Sulphate, or gypsum, occurs in large quantities in Nature. The -salt contains 20·9 per cent. of combined water, and when carefully -heated to 120° C, it loses about two-thirds of this water, yielding a -white powder known as plaster of Paris. This substance, when made into a -paste with water, gradually sets to a hard mass, because the partially -dehydrated gypsum re-combines with the water. - -Lead Sulphate, the chief impurity of commercial oil of vitriol, is a -white powder which is very often used for making white paint in place of -lead carbonate (white lead). The sulphate has the advantage over the -carbonate in not being so readily discoloured; its disadvantage is that -it lacks "body." - -Copper Sulphate, or blue vitriol, is frequently found in the drainage of -copper mines, where it is formed by the oxidation of copper pyrites. It -is made on a large scale by roasting sulphide ores of copper in a -current of air. Oxygen combines with copper sulphide, forming copper -sulphate, which is extracted with water and crystallized. It forms large -blue crystals containing 36 per cent. of water. This salt is put to many -different uses. Very large quantities are used for dyeing and calico -printing; some of the green pigments, such as Schweinfurt green, are -made from it. - - - - - CHAPTER III - NITRIC ACID AND NITRATES - - -Nitric acid, the _aqua fortis_ of the alchemists, must be placed next to -sulphuric acid in the scale of relative importance, because of the -variety of its uses. It is indispensable for making explosives, and is -used for the preparation of drugs and fine chemicals, including the -coal-tar dyes. The acid also dissolves many metals, forming nitrates, -which are put to several uses. Silver nitrate is the basis of marking -ink, and it is also the substance from which the light-sensitive silver -compounds required for the photographic industry are made. The important -pigments, chrome yellow and chrome red, are prepared from lead nitrate. -The solvent action of nitric acid on copper is made use of in etching -designs on copper plates. Over and above all this, it must be mentioned -that an adequate supply of "nitrate" is required for artificial manure. -Thus it can be said that with the uses of this acid and its salts are -associated our supply of daily bread, our freedom from foreign -oppression, and many of the refinements and conveniences of life. - -We shall begin the study of nitric acid by taking stock, as it were, of -the natural sources of supply. The free acid is not found in Nature -except for very small traces in the air after thunderstorms. We have, -therefore, to rely entirely on that which can be obtained artificially. -Until quite recently, it could be said that there was only one method of -making the acid, namely, by the distillation of a mixture of potassium -or sodium nitrates and concentrated sulphuric acid. Now, however, nitric -acid is being made from the air, though as yet only in small quantity, -notwithstanding the great development of this method owing to war -requirements; hence, we are still mainly dependent on the naturally -occurring nitrates just mentioned. - -Potassium Nitrate (nitre, saltpetre, sal prunella) is found in the soil -of hot countries, especially in the neighbourhood of towns and villages -where the sanitary arrangements are primitive. In very favourable -circumstances, it may even appear as a whitish, mealy efflorescence on -the surface of the ground. To obtain the salt, it is only necessary to -agitate the surface soil with water and, after the insoluble matter has -settled down, to evaporate the clear solution. - -Potassium nitrate is required for making gunpowder, which, until quite -recent times, was the only explosive used in warfare. Continental -countries that could not afford to rely entirely on sea-borne nitre had -to make their own. The refuse of the farmyard, mixed with lime and -ashes, was made up into a heap of loose texture, which was periodically -moistened with the drainage from the stables. In the course of years, -saltpetre and calcium nitrate were formed in the surface layers, from -which they were extracted from time to time. The farmer was then allowed -to pay part of his taxes in nitrates. - -Sodium Nitrate, also called caliche, Chili-saltpetre, or Chili-nitrate, -comes mainly from South America. The beds extend for a distance of about -220 miles in Chili, Peru, and Bolivia, between the Andes mountains and -the sea. The deposit is about 5 ft. thick, and its average breadth 5 -miles. The crude material is treated with water in steam-heated wooden -vats. The clear solution is evaporated, and the residue obtained is -washed with the mother liquor and dried. This product may contain as -much as 98 per cent. of the nitrate. - - [Illustration: Fig. 5. PREPARATION OF NITRIC ACID] - -Nitric Acid. Chili-nitrate is always used for making nitric acid. It is -the more abundant of the two naturally occurring nitrates, and therefore -cheaper; moreover, weight for weight, it yields more nitric acid than -the corresponding potassium compound. A mixture of sodium nitrate and -sulphuric acid is heated in a large cast-iron retort (C, Fig. 5). The -retort is entirely surrounded by flame and hot gases to prevent the -condensation of the acid on the upper parts. If this precaution were not -taken, the acid would dissolve the iron and the life of the retort would -not be long; moreover, the product would contain ferric nitrate as an -impurity. The vapour of the acid is led away by the tube D into a series -of two-necked earthenware receivers called _bonbonnes_ (E), and there -condenses to a liquid. The lower figure shows how the leading tube of -the retort is protected from corrosion by the clay tube _a_, _b_; and -how it is connected to the first receiver by the glass tube _e_, which -is luted on at _f_. The percentage strength of the acid which distils -over depends upon that of the sulphuric acid used and on the purity of -the sodium nitrate. - -Pure nitric acid is a colourless liquid 1·559 times as heavy as water, -volume for volume. It fumes strongly in air, and is a very corrosive -liquid. The pure acid of commerce is obtained by distillation of a less -concentrated acid. It is 68 per cent. pure. It is rendered free from -dissolved oxides of nitrogen by blowing air through it. When kept -exposed to light, the colour changes at first to yellow and then to -brown, because light causes a certain amount of decomposition. - -Red fuming nitric acid owes its colour to the great quantity of oxides -of nitrogen dissolved in it. It is made by distilling sodium nitrate -that has been thoroughly dried with the strongest sulphuric acid; the -distillation is carried out at a high temperature, with the express -purpose of decomposing some of the nitric acid to furnish the oxides of -nitrogen. Sometimes a little powdered starch is also added to facilitate -the formation of these oxides. This variety of nitric acid is -particularly active and is used in many operations, especially in making -dyes, explosives, and other organic chemicals. - -Nitric acid has all the general properties of an acid, that is, it has a -sour taste even in very dilute solution, it changes the colour of litmus -to red, and dissolves carbonates and many metals. - -When the vapour of nitric acid is passed through a red-hot tube, and -also when a nitrate is strongly heated, oxygen gas is given off. -Analysis shows that the oxygen combined in pure nitric acid amounts to -76 per cent. of its weight, while that in sodium and potassium nitrates -is 56 and 50 per cent. respectively. Nitric acid and the nitrates are, -therefore, highly oxygenated compounds; moreover, under favourable -circumstances, they are rather easily broken up. - -Pure nitric acid will set fire to warm, dry sawdust, and a piece of -charcoal or sulphur thrown on the surface of molten nitre takes fire -spontaneously and is quickly consumed, giving out a very vivid light. -The explanation of this is that the supply of oxygen is abundant; it is -also readily available and concentrated in a small space. We can vary -the experiment. When a mixture of 75 parts by weight of finely-powdered -saltpetre, with 15 of charcoal dust and 10 of ground sulphur, is -ignited, it burns very vigorously, and is soon consumed. This mixture -is, indeed, home-made gunpowder. - -Explosives. Gunpowder was discovered in very early times by the Chinese, -but for many years the secret of its composition did not get outside the -Great Wall. In the fifth century A.D., it was apparently re-discovered -at Constantinople, and that city was for a long time defended by the use -of what is known in history as Greek Fire, an incendiary mixture very -similar to, if not actually the same as, gunpowder. But again the secret -of its composition was jealously guarded, and it was not until the -thirteenth century that it was discovered, apparently for the third -time, and introduced to Western Europe by Roger Bacon. It was used in -siege cannon early in the fourteenth century and in field guns at Crécy; -but it was apparently not employed for blasting until about 1627, -although in 1605, Guy Fawkes and his fellow-conspirators were able to -obtain it in large quantity. - -From the battle of Crécy in 1346 to the beginning of the South African -campaign in 1889, gunpowder was the only explosive used in warfare. -"Villainous saltpetre" has therefore played a very important part in -shaping the course of events in the world's history. At the present day, -gunpowder has become "old-fashioned." In warfare, it has been superseded -by "smokeless" powders of much greater power; while for mining -operations, explosives with a much greater shattering effect have long -since taken its place. - -The composition of gunpowder may vary, but on the average it contains 75 -parts by weight of saltpetre to 15 of charcoal and 10 of sulphur. It is, -therefore, a mixture of two combustible substances, with a large -quantity of a third very rich in oxygen. The separate constituents are -very finely ground and afterwards thoroughly incorporated. When the -mixture is ignited, charcoal and sulphur burn very fiercely in the -oxygen supplied by the saltpetre. - -The secret of the action of gunpowder lies in the extraordinary rapidity -with which combustion, started at one point, is propagated through the -whole mass. Moreover, the products of combustion are mainly gases, and -these occupy several thousand times the volume of the solid from which -they are produced. In a confined space, a gas may exert enormous -pressure when its normal tendency to expand is resisted. - -Propellants. Although combustion is propagated through a quantity of -gunpowder with very great rapidity, it is not done instantaneously. The -time required is about one-hundredth of a second under ordinary -conditions, and this interval, short though it is, is very important. -When the object is to throw a projectile, the inertia of the latter has -to be overcome, that is, a certain amount of force has to be applied -before the heavy body begins to move. In order that the strain on the -breech of the gun may be as small as possible, the pressure must be -gradually developed and must reach its maximum just as the projectile -begins to move. - -The time factor in the explosion constitutes the difference between what -we now call "propellants" and "high explosive." Propellants are -explosives which develop pressure gradually, and are therefore used to -launch the projectile; high explosive develops pressure instantaneously, -and is therefore used as the bursting charge inside the shell, bomb, or -torpedo, and also in blasting operations. - -Cordite, or smokeless powder, is the propellant now most used. It is -made by macerating guncotton and nitroglycerine with their common -solvent acetone. A pulp is thus made to which 5 per cent. of vaseline is -added. The mixture is then forced through a die, and in this way it is -formed into threads or rods, which harden as the acetone evaporates. -Cordite produces no smoke, because all the products of its combustion -are invisible gases. - -High Explosive. _Nitroglycerine_ and _Guncotton_ are both explosives of -the instantaneous kind. The former is made by forcing glycerine, under -pressure in a very fine stream, into a mixture of fuming nitric and -concentrated sulphuric acids; the latter by soaking cotton-wool in a -similar mixture. Both products are washed with water until quite free -from acid, and subsequently dried. - -Nitroglycerine is a colourless oil with a burning taste. The oil itself -is very dangerous to handle, for it is liable to explode spontaneously -even when the utmost care has been taken in its preparation. A mere spot -on a filter paper explodes with a deafening report when gently hammered -on an anvil; and one drop, when heated on a stout iron plate, blows a -hole through the plate. No use could be made of this substance for many -years after its discovery because it was so liable to explode during -transportation; now, however, it is made safer by mixing with absorbent -infusorial earth or _kieselguhr_. This mixture is known as dynamite. -Blasting gelatine, like cordite, is a mixture of nitroglycerine and -guncotton. - -_Trinitrotoluene_ (T.N.T.) is made from toluene and nitric acid, and is -a type of the modern high explosive. It is a yellow crystalline -substance which melts at 79°-81·5° C., and is poured into the shell in a -molten condition. It is a remarkably stable substance, which burns -quickly when heated to 180° C.; it cannot be exploded even by hammering. -Explosion is only brought about by that of a subsidiary substance called -the detonator. The percentage composition of T.N.T. is as follows-- - - Carbon 33·5 - Hydrogen 2·3 - Nitrogen 19·5 - Oxygen 44·7 - 100·0 - -The oxygen present is only just sufficient to burn the whole of the -carbon to carbon monoxide; but since carbon dioxide is also formed, -which requires twice as much oxygen for the same weight of carbon, and -since the hydrogen and nitrogen may also be oxidized, the combustion of -the carbon is not complete; and therefore the explosion of T.N.T. is -accompanied by a dense black smoke, consisting of finely divided -particles of carbon. - -The explosive known as ammonal is a mixture of T.N.T., aluminium powder, -and ammonium nitrate; the function of the latter substance is to supply -more oxygen to render the combustion of the carbon of T.N.T. complete. - -Nitrates and the Food Supply. Chemical analysis shows that compounds of -nitrogen enter largely into the composition of the living tissues of all -plants and animals; hence, either nitrogen itself or some of its -compounds must be assimilated by all living organisms to provide for -growth and development, and to repair wastage. Air, since it contains -approximately four-fifths of its volume of free nitrogen, is the most -obvious source of supply. At every breath, a mixture of oxygen and -nitrogen is inhaled by animals, but only part of the oxygen is used. -Practically the whole of the nitrogen is returned to the atmosphere -unchanged; it serves only to dilute the oxygen. From this it is clear -that animals do not build up their nitrogenous constituents from -elementary nitrogen. - -With plants it is very much the same, for, although they obtain their -principal food, namely, carbon, from the carbon dioxide which is present -in air, it is only in a few exceptional cases that free nitrogen is -assimilated. The exceptions will be considered first, because it was -through these that we first began to learn something definite about the -great importance of nitrogen in agriculture. - -Virgil, who was born in 70 B.C., wrote a poem in praise of agriculture. -Almost in the opening lines he deals with the treatment of corn land. He -advises that, in alternate years, this should either be left fallow or -sown with pulse, vetch, or lupin; but not with flax or oats, because -they exhaust the land. From this we learn that rotation of crops was one -of the established principles of good husbandry even at the beginning of -the Christian era. - -It was not until the later years of the nineteenth century that any -explanation as to why rotation of crops is beneficial was put forward. -Let us first state the facts more precisely. Peas, beans, vetches, -clover, and other members of the natural order called _Leguminosae_, -which includes about 7,000 species, produce fruits rich in complex -nitrogen compounds without being dependent in any way upon nitrogen -compounds in the soil. Moreover, they do not exhaust the land as far as -these compounds are concerned; hence wheat and other grain can be grown -on the same land the following year. - -It is now known that leguminous plants assimilate atmospheric nitrogen -with the help of certain bacteria. If anyone will dig up a lupin root, -he will observe[2] conspicuous wrinkled swellings or nodules at various -points on the roots. These, when examined with a high-power microscope, -are found to contain colonies of bacteria. It is these minute vegetable -organisms which assimilate nitrogen and pass on nitrogen compounds to -the larger plant. Other plants cannot assimilate what we might call raw -nitrogen; they require soluble nitrates. These they build up into -complex organic nitrogen compounds suitable for the feeding of animals -which can assimilate neither free nitrogen nor nitrates. - -The Nitrogen Cycle. The supply of nitrates in the soil needs continually -to be renewed by the addition of decaying vegetable matter, stable or -farmyard manure, or Chili saltpetre. The natural manures contain organic -nitrogen compounds which were built up during the life of some animal or -plant. They are not immediately available as food for other plants, -because they are, as it were, the end products of life, and are not -soluble in water. But Nature provides for this. The manures decay, -forming humus, and ultimately ammonia, one of the simplest of inorganic -nitrogen compounds. Ammonia is then transformed to nitrites by certain -bacteria present in the soil, while other bacteria change nitrites into -nitrates. Both of these organisms are quite distinct from the root -nodule bacteria of the _Leguminosae_. - -The nitrates pass into the plant in solution, and then begins again that -wonderful cycle of changes which we have described. This is perhaps made -clearer by the following diagram. - - [Illustration: Fig. 6. THE NITROGEN CYCLE] - -It now remains to show why artificial manures also are necessary. Let us -consider what happens to a piece of ground which is left uncultivated. -Although nothing is taken from it in the way of a crop, yet it very -quickly deteriorates, and the soil becomes infertile through the loss of -nitrogen compounds. This is explained by the fact that nitrates are -soluble in water, and so they get washed away from the top soil. In -addition to this, the nitrogen which is returned to the land forms quite -an insignificant fraction of that which is taken from it, for we waste a -great deal of organic nitrogen. The difference on both these accounts -has, therefore, to be made up by the addition of artificial manures -containing soluble nitrates. - -The natural supply of nitrate is very limited. According to a report of -the Chilian Government published in 1909, the nitre beds of that country -were expected to last for less than a century at the current rate of -consumption. Wheat, above all things, will not grow to perfection on -soil which is deficient in nitrate. In 1908, Sir William Crookes called -attention to the difficulty which might be experienced in the near -future in supplying the people of the world with bread. Statistics -showed that wheat was grown on 159,000,000 acres out of a possible -260,000,000. The average yield is 12·7 bushels per acre. By 1931, it is -calculated that the population of the world will be 1,746,000,000; and -to supply these with bread, wheat would have to be grown on 264,000,000 -acres, that is, 4,000,000 acres beyond the total available wheat land. - -The remedy which Sir William Crookes suggested in order to avoid famine -was to raise the average yield from 12·7 to 20 bushels per acre by the -application of an additional 12,000,000 tons of Chili saltpetre per -annum. In view of the possible exhaustion of the supply of this -substance, this would only mean a postponement of the evil day. The -position, however, is now modified to a great extent because undeveloped -deposits of sodium nitrate are known to exist in Upper Egypt, and the -making of nitric acid from the air, which in 1908 was put forward as a -suggestion, is now an accomplished fact. - -Nitric Acid from Air. The supply of nitrogen in the air is truly -inexhaustible; it amounts to about 7 tons for every square yard of the -earth's surface, which is about 200,000,000 square miles. It is quite -evident that anything man may do in the way of taking nitrogen from the -air will make no perceptible difference to its composition. - -Every time a flash of lightning passes between a cloud and the earth, -oxygen and nitrogen combine in the path of the spark, producing oxides -of nitrogen. These dissolve in water, and are washed into the earth as a -very dilute solution of nitric acid. As long ago as 1785, H. Cavendish -imitated this natural phenomenon. A reference to the diagram (Fig. 7) -will show how nitric acid can be made from the air on a small scale. The -globe contains air under slightly increased pressure. The platinum wires -or carbon rods are connected with the terminals of an induction coil, -which in its turn is connected to accumulators supplying the current -required. - -When the coil is put into action, a spark passes across the gap between -the ends of the carbon rods. With a larger coil and a more powerful -battery, there is an arching flame which can be blown out and -re-lighted. This is actually nitrogen burning in oxygen. The result in -either case is the same; the air in the globe sooner or later acquires a -reddish-brown colour due to oxides of nitrogen, which, when shaken with -water, form a very dilute solution of nitric acid. - -The same process is now carried out on a large scale. Air is driven by -fans through a very powerful electric arc, whereby 1·5 to 2 per cent. is -converted into nitric oxide. This combines spontaneously with more -oxygen to form nitrogen peroxide, which, when dissolved in water, gives -a very dilute solution of nitrous and nitric acids. - - [Illustration: Fig. 7. NITRIC ACID FROM AIR] - -The absorption of the oxides of nitrogen is carried out systematically. -The mixed gases, after passing through the arc, are passed through a -series of towers filled with acid-resisting material over which a stream -of water is flowing. The solution of nitric acid so obtained is very -dilute, but by using the liquid over and over again, a moderately strong -solution is ultimately produced. This is collected in granite tanks and -neutralized with lime, forming calcium nitrate or Norwegian saltpetre, -as it is now called. - -This is a new industry and a rapidly-growing one; in the course of five -years (1905-1909) the annual output of Norwegian or "air" saltpetre -increased from 115 to 9,422 tons. Mountainous countries like Norway and -Switzerland are perhaps in a specially favoured position with respect to -this industry. Rapid streams and waterfalls, in conjunction with -turbines, are used for driving the dynamos, and in this way electricity -is produced at very low cost. It is interesting, however, to note that a -plant for the manufacture of nitric acid from air has now been -established in Manchester. - - - - - CHAPTER IV - THE HALOGEN ACIDS - - -A group of acids, namely, hydrochloric, hydrofluoric, hydrobromic, -hydriodic, must now be considered together with their corresponding -salts. In appearance and in other physical properties they resemble one -another very closely; they are, therefore, called by the general name -"halogen acids." This name is derived from the Greek word meaning -"sea-salt," which is a mixture of the salts of these acids, and from -which the acids themselves can be obtained by treatment with oil of -vitriol. - -Hydrochloric Acid. When concentrated sulphuric acid is added to common -salt, a gas is liberated which has a very pungent acid smell and taste. -This is a compound of the elements hydrogen and chlorine, and therefore -called hydrogen chloride. It is extremely soluble in water; a given -volume of water dissolves as much as 500 times its own volume of the -gas. The solution produced in this way is now called hydrochloric acid, -but formerly it was known as spirits of salt, or muriatic acid. - -Hydrochloric acid has all the general properties of acids. It dissolves -many metals, such as zinc, iron, aluminium, and magnesium; hydrogen gas -is given off, and the chloride of the metal is formed. It also dissolves -limestone, marble, and all forms of calcium carbonate; carbon dioxide -gas is liberated, and a solution of calcium chloride remains. - -The hydrochloric acid of commerce is obtained as a by-product in the -manufacture of washing soda from common salt by the method proposed by -Nicholas Leblanc towards the end of the eighteenth century. In the first -stage of this process, salt is mixed with sulphuric acid; this causes -the liberation of hydrogen chloride gas, which, when dissolved in water, -produces hydrochloric acid. - -The past history of this branch of chemical industry is interesting. -Until about 1870, there was no very great demand for hydrochloric acid, -and in the early days of the working of the Leblanc process the soda -manufacturer took no pains to recover more than he could actually sell. -Consequently, a large quantity of hydrogen chloride gas was allowed to -escape into the air, with results which can well be imagined. For miles -around, great damage was frequently sustained by the growing crops; when -it rained in the neighbourhood of the works, the gas was washed out of -the air and, speaking quite literally, it rained dilute hydrochloric -acid, which rapidly corroded all stone and metal work. It is not, -therefore, surprising to learn that alkali makers were frequently -involved in litigation, and chemical works were regarded as a great -nuisance. - -By the Alkali Act of 1863, chemical manufacturers were compelled to -prevent the escape of more than 5 per cent. of hydrochloric acid gas; -and by a subsequent Act, this limit was lowered to 0·2 grain per cubic -foot. The provisions of the Acts were not difficult to carry out, -because hydrogen chloride is extremely soluble in water. - -The gases coming from the pans in which the salt was decomposed were led -into towers (see Fig. 8) built of bricks or Yorkshire flags soaked in -tar. These towers were filled up with coke or other acid-resisting -material, which was kept moist by water flowing from the tank F. In this -way, hydrogen chloride gas was removed and hydrochloric acid collected -in tanks (not shown in the figure) at the bottom of the towers. Even -then, there was no market for the greater part of the recovered acid, -consequently much of it found its way into drains and streams, and so -carried on its work of destruction in a less obtrusive way. - - [Illustration: Fig. 8. PREPARATION OF HYDROCHLORIC ACID] - -By another piece of legislation, which at first sight seems to be wholly -unconnected with Chemistry, hydrochloric acid acquired a greatly -enhanced value. In 1861, the tax on paper was removed, and in the next -twenty years the demand for that commodity increased so much that raw -material both cheaper and more abundant than rag had to be found. -Esparto grass and eventually wood pulp proved successful substitutes. -There is really very little difference in composition between cotton and -linen rag on the one hand and wood fibre on the other, for both are -mainly composed of cellulose, which is a definite chemical compound. -Wood fibre is the less pure, and it is also coloured, and therefore has -to be bleached before it can be used for making white paper. It was this -circumstance which led to the greatly increased demand for hydrochloric -acid. - -At the beginning of this chapter, it was mentioned, in passing, that -hydrogen chloride gas is a compound of hydrogen and chlorine. The latter -element is a very active bleaching agent, and is most easily obtained by -treating hydrogen chloride or its solution in water with pyrolusite -(black oxide of manganese), whereby the hydrogen is oxidized, forming -water, and chlorine gas is set free. Being a gas, chlorine is not -convenient to handle in large quantities; it is, therefore, converted -into bleaching powder, commonly but wrongly called chloride of lime. - -Bleaching Powder. The manufacture of bleaching powder is carried out in -the following way. Slaked lime to the depth of 3 or 4 in. is spread over -the floor of a special chamber which can be made gas-tight. The lime is -raked up into ridge and furrow, and the chamber is filled with chlorine. -At the end of about twenty-four hours, the greater part of this chlorine -will have been absorbed by the lime. The chamber is then opened, the -lime is raked over to expose a fresh surface, and the process of -chlorination is repeated. Generally this is sufficient; the bleaching -powder should then contain about 35 per cent. of available chlorine. - -The demand for bleaching powder is great and steadily increasing. The -price of 35 per cent. bleaching powder has never been less than about £5 -a ton,[3] so that it is perhaps unnecessary to add that the absorption -of hydrogen chloride gas is now made so complete that it is well within -the requirements of the second Alkali Act. - -Chlorides. The salts of hydrochloric acid are called chlorides, and the -most important of these is sodium chloride or common salt--a body that -is so well known that it need not be described here. - -Although the quantity of this substance required for domestic purposes -is very large, it is, nevertheless, small by comparison with that which -is used for industrial purposes. It has already been mentioned that salt -is the starting-point for the manufacture of washing soda by the Leblanc -process, and, in addition to this, it is employed in the glass industry -to produce whiteness and transparency in certain kinds of glass; in -pottery, for glazing earthenware; in soap-making, for salting out the -crude soap; and in the dye trade as a mordant, and also for improving -the quality of certain colours. A full account of the salt industry is -given in another volume of this series. - -Hydrofluoric Acid. When calcium fluoride (fluorspar, Derbyshire spar, or -blue-john) is warmed with concentrated sulphuric acid in a leaden dish, -hydrogen fluoride gas is evolved, and this, when dissolved in water, -gives hydrofluoric acid. - -The peculiar property of this substance is that it has a very marked -corrosive action on glass. It cannot, therefore, be kept in glass -vessels, but must be stored in bottles made of hardened caoutchouc. On -the other hand, it is this same property which gives it its place in -commerce. As far back as 1670 it was used for etching on glass. The -process is a very simple one. The article is first coated with wax, -which is then removed in places by a sharp pointed tool. When exposed to -the action of the gas or its solution, corrosion takes place only where -the glass has been laid bare, the other parts being protected by the -wax. After a short interval, the wax can be melted off, and the design -made more distinct by rubbing in some opaque cement. For general trade -purposes, such as the stamping of lamp chimneys or electric light bulbs, -a quicker method is required. In this case, a preparation of -hydrofluoric acid which can be applied with a rubber stamp is used. - -Fluorspar or calcium fluoride is the most important salt of hydrofluoric -acid. It is a commonly occurring mineral, and besides its use for the -preparation of the acid, it is employed in many metallurgical operations -to form a fusible slag. - -Hydrobromic and Hydriodic Acids are not much used, but their salts, the -bromides and iodides respectively, are of great technical importance. -Silver chloride, bromide, and iodide, are sensitive to light, and mixed -with gelatine they form the emulsion which is spread over photographic -plates and papers. Potassium bromide and iodide are also well known to -photographers. - -When the halogen salts of silver are exposed to light, an extremely -subtle chemical change takes place, which is only made apparent when the -plate or paper is developed. Then the silver salts on which the light -has fallen are reduced to metallic silver, and this reduction is -greatest where the light was most intense, and in other places is -proportional to the light intensity. A very faint image may appear on -the plate while it is in the developer, but generally the image is only -brought out clearly when the plate, film, or paper is placed in "hypo" -solution, which dissolves out the silver salts which have not been -changed, leaving the metallic silver unaffected. - - - - - CHAPTER V - CARBONIC ACID AND CARBONATES - - -Carbon. When any product of animal or vegetable life is strongly heated -in a vessel from which all air currents are excluded, a mixture of gases -and liquids is driven off, and a charred mass remains. This residue, -from whatever source obtained, is composed mainly of the element carbon. -It sometimes happens that a loaf of bread or a cake is left in the oven -and forgotten. In popular language it is then said to be "burnt to a -cinder"; in reality, the surface layers have been converted into carbon. - -Carbonic Acid. If carbon is heated in an open vessel provided with a -good draught, it glows and in time disappears, because it combines with -oxygen to form an invisible gas, carbon dioxide or carbonic acid gas, -which, when dissolved in water, forms carbonic acid. - -Compared with the acids which have been described in the foregoing -chapters, this is a very feeble acid; it changes the colour of litmus to -a wine red, not a bright pink; its taste is just pleasantly acid, and -its solvent action on metals and limestone is very small indeed. The -solution of the acid, obtained by passing carbon dioxide into water, is, -of course, very dilute, and it cannot be concentrated by evaporation, -since this only results in expelling the carbon dioxide from solution, -leaving pure water. - -Soda Water. In the case of most gases, the weight which dissolves in a -given quantity of water is proportional to the pressure. This is true -for carbonic acid gas. Under a pressure of 4 atmospheres, the weight of -gas which dissolves is four times as great as under a pressure of one -atmosphere. - -Soda water is water charged with carbon dioxide under pressure. This -pressure is maintained from the time it leaves the manufacturer to the -time it reaches the consumer by the strong walls of the syphon or -bottle. Immediately this pressure is released, the greater part of the -excess gas escapes, producing effervescence. It is, however, curious to -note that all the gas which ought to escape when the pressure is reduced -does not do so at once. If soda water is allowed to stand in an open -glass until it becomes "flat," a brisk effervescence can be started -again by dropping a lump of sugar into the quiescent liquid. Soda water -remains supersaturated with gas for some time after the pressure has -been released. - -Calcium Carbonate. The salts of carbonic acid are called carbonates. -Calcium carbonate is one of the most abundant substances in Nature. The -white cliffs of the east and south coasts of England, and those of -France across the intervening sea, are the exposed parts of enormous -beds of chalk or calcium carbonate. Whole mountain ranges in various -parts of the world are composed of limestone, which in some cases is -mainly calcium carbonate, and in others a mixture of this substance with -magnesium carbonate. Marble, whether white, black, or variegated, is -almost pure calcium carbonate, the differences of colour being due to -insignificant traces of iron and other foreign matter. In Iceland spar -and calc spar, sometimes called dog-tooth spar, we have two transparent -crystalline forms of this same substance. - -Connected with the animal kingdom there are forms of calcium carbonate -no less varied in appearance. Egg shells are composed of this substance, -and so are oyster shells and the hard external coverings of some of the -lower animals. The mother-of-pearl lining of the oyster shell, and also -the pearl itself, are secretions of calcium carbonate. The beauty of the -last-named variety is due to the external form and to minute -inequalities of the surface, which cause the resolution of white light -into colours seen in the spectrum or in the rainbow. The coral reefs or -_atolls_ of the Southern oceans, which may be miles in breadth and -hundreds of miles in length, are all composed of calcium carbonate, -which a tiny marine animal has formed for its own support and -protection. - -It is perhaps somewhat surprising at first to be told that all these -forms are composed of the same chemical substance, yet on this point the -evidence is definite and unmistakable. All the varieties dissolve -readily in dilute hydrochloric acid with effervescence caused by the -escape of carbon dioxide gas; moreover, if any of the purer forms, such -as pearl, marble, or Iceland spar, are heated to redness for some time, -they all lose about 44 per cent. by weight, leaving a residue which is -pure lime. - -Quicklime. The making of lime from limestone or chalk is called lime -burning. The operation is carried out in a structure called a lime kiln, -which is usually a barrel-shaped vertical shaft surrounded by -substantial brickwork. There are two main methods of procedure, the one -continuous and the other intermittent. In the continuous process, the -kiln is filled up with limestone and fuel (generally coke) in alternate -layers. Combustion is started at the bottom and maintained by a -regulated draught. As the charge works down, the addition of limestone -and fuel is continued from the top, while the lime is removed from the -bottom of the kiln. The lime produced by this method has the ashes of -the fuel mixed with it. To avoid this, the more modern type of kiln has -four lateral fire grates outside the actual kiln. - -For the intermittent method, a kiln is required which has a fireplace at -the bottom. Over this a rough arch is built of large pieces of -limestone, laid dry, and then the kiln is filled up with pieces of -limestone which decrease in size from below upwards. The fire is kindled -beneath the arch and urged by a regulated draught. The heating is -maintained for three days and nights, after which time the charge is -allowed to cool down. - -Carbonic Acid Gas in Nature. Although the solvent action of carbonic -acid is very small compared with that of strong acids, it is -nevertheless great in comparison with that of water. This is shown -especially in its action on limestone, an action from which several -important consequences arise. Rain, as it falls through the air, -dissolves a little carbon dioxide and, although this is only an -exceedingly dilute solution of a very weak acid, its cumulative effect, -especially in limestone districts, is very great; it hollows out -enormous caves and causes the formation of those fantastic creations in -stone known as stalactites and stalagmites. - -When a drop of water charged with carbonic acid gas falls on limestone, -it dissolves a little of that substance, forming calcium bicarbonate, -which may be regarded as a compound of calcium carbonate, carbon -dioxide, and water. Little by little, the solid rock is hollowed out and -a cave, or perhaps an underground watercourse, is formed. - -Again, the drop of water charged with calcium bicarbonate may find its -way to the roof of a cave. As it hangs from the roof while it gathers -strength to fall, a little of the carbon dioxide escapes, and a minute -quantity of calcium carbonate is deposited. In this way, a stalactite -looking like an icicle in stone gradually grows downwards. - -When the drop reaches the floor of the cave, a little time elapses -before it sinks into the ground; again a little carbon dioxide escapes, -and a small quantity of calcium carbonate is formed. Little is added to -little, and in the course of ages the stalagmite grows upward from the -floor and ultimately meets the stalactite to form a continuous column of -glistening crystallized calcium carbonate. - -Hard and Soft Water. Water that is used for domestic or manufacturing -purposes is described as either hard or soft. Soft water produces a soap -lather almost at once; hard water forms at first a scum or curd which -has no detergent properties, and only after a time gives the soap lather -which is required. The difference is due to the relative amount of -dissolved solid contained in the water. - -Only distilled water or rain water collected in the open country is -perfectly soft, for this is the only kind of water which on being -evaporated to dryness leaves no solid residue. In districts where the -underlying strata are composed of hard insoluble rock, such as granite -or millstone grit, the water contains very little dissolved matter and -is relatively soft. In a limestone or chalk country, water is very hard -and in many cases has to be softened either before delivery or before -use. - -The chief impurities which cause hardness are the chlorides, sulphates, -and bicarbonates of magnesium and calcium. The chlorides and sulphates -are not affected in any way by boiling, and the hardness which is due to -them is said to be "permanent." The bicarbonates, on the other hand, are -decomposed when the water is boiled, and then they cease to cause the -water to be hard. This part of the hardness is spoken of as "temporary" -hardness. - -Let us now consider what calcium bicarbonate is and how it is formed. It -is a compound of calcium carbonate and carbonic acid, and is formed by -the solvent action of carbonic acid on limestone or chalk. The compound -is soluble in water; but when the solution is boiled, the carbonic acid -is broken up, carbonic acid gas is expelled from the solution, and -calcium carbonate is formed. - -Temporary hardness is the more troublesome. In the first place, the -bicarbonates, especially that of calcium, often form the greater part of -the dissolved impurity. Moreover, when the water is boiled, although the -hardness is removed, the insoluble calcium carbonate is a source of -trouble, for it gradually settles down into the hard mass known as "fur" -in kettles and "scale" in boilers. - -It is perhaps necessary at this point to emphasize the fact that matter -_suspended_ in water does not make it hard, and it is only matter which -is _dissolved_ which makes any difference in this respect. - -Since the softening of temporary hard water by boiling has the -undesirable feature of introducing solid matter into the boiler, it is -customary now to treat this water chemically. The following is the -process most generally used. Quicklime or slaked lime is stirred into -the water until the mixture gives a faint brown coloration when a drop -of silver nitrate is added to a small test portion. Unsoftened water is -then added until a sample just ceases to give this test. The temporary -hardness has then been removed, and it is only necessary to allow the -suspended matter to settle. - -The explanation of the method is as follows. The lime which is added -neutralizes the carbonic acid combined with the calcium bicarbonate, and -the result is the same as in the former case where this carbonic acid -was decomposed by heating, for calcium carbonate is thrown out of -solution. - -For domestic purposes, water is softened by the addition of washing -soda. Since this reacts with all the calcium and magnesium compounds -forming the insoluble carbonates, all hardness, both temporary and -permanent, is removed. - - - - - CHAPTER VI - PHOSPHORIC, BORIC, AND SILICIC ACIDS - - -The acids which are grouped in this chapter are not in themselves of -much interest, though some of their salts are extremely important -compounds. - -Bone. Much of the refuse bone, sooner or later, reaches the marine -store, and from that point starts on a career of usefulness in the -industrial world. - -"Green bone," as it is then called, may have fat adhering to it or -confined in its hollow interior as marrow. This is recovered by -treatment with benzine, and after that the bone is subjected to the -action of superheated steam in order to convert cartilage into glue. In -some cases, the residue is then ground up to make bone meal, which is -valuable as a manure because of the calcium phosphate which it contains. -In this way, the phosphate returns again to the animal kingdom, for it -supplies plants with the phosphates that they require, and from the -vegetable kingdom it passes to animals and helps to build up bone again. - -Calcium Phosphate and Bone Black. Instead of being ground up, bone may -be heated in a retort in much the same way as coal is treated for the -manufacture of coal gas; bone oil is distilled off, and a non-volatile -residue, called bone black or animal charcoal, remains. This contains -about 90 per cent. of calcium phosphate and 10 per cent. of finely -divided carbon disseminated throughout the mass. It has the peculiar -property of absorbing colouring matter, and is used for this purpose in -the sugar industry and in the preparation of fine chemicals. - -Phosphoric Acid. After being some time in use, bone black loses the -property of absorbing colouring matter; and though it can be "revived" -several times by heating it strongly in a closed retort, it ultimately -becomes spent and of no further use to the sugar refiner. It is then -heated again, this time in an open vessel, until all the carbon is burnt -away. The residue is now a greyish solid consisting mainly of calcium -phosphate. This, supplemented with native phosphate, which is probably -fossilized bone, is used for the preparation of phosphoric acid. - -The salt is decomposed by sulphuric acid in wooden vats; calcium -sulphate is formed, and ultimately settles on the bottom of the vat, -leaving a clear supernatant liquid, which is a dilute solution of -phosphoric acid. This liquid is drawn off and evaporated to a syrup. -This is "syrupy" phosphoric acid. On being still more strongly heated, -the syrup loses still more water, and a semi-transparent glassy-looking -substance, called metaphosphoric acid, remains. - -Superphosphate. All fertile soils, especially those on which wheat is to -be grown, must contain a certain amount of phosphate. With this, as with -all other plant foods, the actual percentage weight required in the soil -is very small indeed, but it is necessary that it should be disseminated -throughout the soil. Even distribution is very difficult to secure in -the case of a substance like calcium phosphate, which is practically -insoluble in water. - -To get over this difficulty, calcium phosphate is converted into a -mixture known as "superphosphate" by the following process. Bone ash or -the mineral phosphate is finely ground and thoroughly mixed by machinery -with two-thirds its weight of sulphuric acid from the lead chambers. -After a time, this mixture sets to a hard mass, containing principally -gypsum and calcium tetrahydrogen phosphate. It is then ground up finely -and is ready for use. - -The special modification of calcium phosphate contained in -superphosphate is soluble in water. It is, therefore, carried into the -soil in solution, and in this way very evenly distributed. In the soil -it reacts with the lime or chalk which is present, and is gradually -reconverted into insoluble calcium phosphate. - -The manufacture of superphosphate is a very important industry. The -weight of the substance produced annually in Great Britain alone is not -far below a million tons. - -Basic Slag. In the Bessemer process for converting iron into steel, cast -iron is melted up in a vessel called a converter and, by the aid of a -powerful blast blown through the molten iron, most of the impurities are -burnt off. If, however, phosphorus and sulphur are present, they are not -removed if the converter has a silica (acid) lining. The original -Bessemer process was, therefore, modified by Thomas and Gilchrist, and -the converter for this kind of iron is lined with dolomite and lime -(basic lining). Phosphorus is then converted into phosphate and retained -by the lining, which is subsequently removed, ground up finely, and sold -as "basic slag." - -Boric Acid, or boracic acid, is familiar because it is used in medicine -as a mild antiseptic; it is also employed as a preservative for food. It -is a white crystalline compound, sparingly soluble in water. It has no -well-marked taste, and causes only a partial change in the colour of -litmus solution; it is, therefore, one of the weak acids. It does not -dissolve metals, but it displaces carbon dioxide from carbonates, -forming salts. - -Borax, the best known salt of boric acid, is used in laundry work and -also for making some enamels, for when it is strongly heated it loses -water, and ultimately melts down to a clear "glass" in which the oxides -of metals will dissolve, yielding transparent substances which are -beautifully coloured according to the nature of the oxide used. This -property is often made use of in chemical analysis in what is known as -the "borax-bead" test. - - [Illustration: Fig. 9. BORIC ACID] - -Boric acid is a natural product; the method by which it is obtained is -of some interest, because it is so simple, and because it shows how mere -traces can be gradually accumulated until a very fair total is -ultimately obtained. Moreover, the method is copied directly from -Nature. - -In the early years of the nineteenth century, certain jets of natural -steam, called _suffioni_, which issue from the earth in Tuscany, were -found to contain the vapour of boric acid. These jets of steam are of -volcanic origin. The quantity of boric acid in the vapour is very small -indeed; nevertheless, by the method which is adopted, it can be -profitably recovered, and more than a ton of the solid is daily -produced. - -In the same country there are many lagoons, the water of which contains -boric acid. It was rightly conjectured that this boric acid came from -jets of steam which issued from the earth in the bed of the lagoon. This -suggested the idea of building up an artificial lagoon around a group of -jets. - -Series of about five of these collecting basins (Fig. 9) are formed, -each one at a slightly lower level than the one which precedes it. The -first basin is filled with water from an adjacent spring, and this is -allowed to remain for twenty-four hours. A sluice is then opened and the -liquid contained in the first basin flows down to the second, where it -remains for another day, and so on until it reaches the last basin of -the series. The liquid by this time is almost fully charged with boric -acid, but it contains only about 2 per cent., because the acid is so -sparingly soluble in water. - -From the last basin (A), the liquid runs into large vats (B, D), where -the suspended impurities settle down. The solution of boric acid is then -concentrated by causing it to flow over a broad inclined plane made of -corrugated lead or through a series of shallow vessels heated by jets of -natural steam. The hot liquid flows into another vat (C), and, as it -cools, boric acid crystallizes out and is removed by perforated ladles. - -The mother liquor from which the crystals have been withdrawn is, of -course, a cold saturated solution of the acid, and this is returned to -the top of the incline to flow down again and lose more water. The boric -acid is finally transferred to drying chambers, which are also heated by -the natural steam. - -Native borax or "tinkal" comes from Thibet and also from Ceylon. In -California, a large quantity of borax is obtained from a borax lake, and -also from the mud of marshes in its neighbourhood. - -Silica. The element silicon does not occur in the free state in Nature, -neither has any particular use been found for it, and therefore it is -not often isolated except to provide a lecture specimen. The compounds -of silicon, however, are both plentiful and important, especially -silica, the oxide, and the silicates or salts of silicic acid. - -The commonest forms of silica are sand, flint, and quartz. Silver sand -is composed of small crystals of pure silica, while flint is the -amorphous variety of the same substance. Quartz, or rock crystal, is a -very hard and transparent mineral. It forms six-sided prisms ending in -pyramids. It is distinguished from other common transparent minerals, -such as calcspar, by the fact that it cannot be scratched even with a -good knife or file, and that a drop of hydrochloric acid has no action -on it. The melting point of silica is very high. - -Sometimes silica is very delicately coloured with minute traces of -metallic oxides and other substances, and these forms, because of their -rarity and beauty, are more highly valued. Smoky quartz, cat's-eye, and -amethyst are some of the coloured varieties of quartz. Opal, agate, -jasper, onyx, and chalcedony are, in the chemist's classification, -merely coloured flints. - -In recent years, chemical apparatus has been made from pure fused -silica. This can only be worked in the oxy-hydrogen blow-pipe flame or -in the electric furnace; nevertheless, crucibles, flasks, beakers, and -retorts can be made. Silica ware has several advantages over glass, -notably, that water has no action upon it at all; moreover, its -coefficient of expansion is so very small that a piece of apparatus made -of silica can be suddenly heated or cooled without risk of fracture; -indeed, it can be made red-hot and cooled immediately by plunging into -cold water. - -Quartz or silica fibres, used for suspending magnets and other bodies in -very delicate physical apparatus, are made in the following way. Molten -silica is attached to the bolt of a crossbow, which is then released -above a carpet of black velvet. As the bolt flies forward, it draws out -the silica into a filament, which is so fine that it would be difficult -to find were it not for the velvet background. - -Silicic Acid itself is only of theoretical interest. It is obtained by -adding hydrochloric acid to a solution of potassium or sodium silicate. -It is a gelatinous substance of somewhat indefinite composition. It has -no effect on litmus, no taste, and no solvent action; in fact, it is -only recognizable as an acid because it dissolves in alkalis, forming -salts called silicates. It is one of the weakest acids known. - -The natural silicates are very abundant and varied; orthoclase or potash -felspar, and albite or soda felspar, are those which most commonly -occur. The former is potassium aluminium silicate, and the latter, -sodium aluminium silicate. Iron is generally present in both as an -impurity. The weathering of the felspars, in conjunction with the action -of water, has produced the clays. In this way, pure white China clay has -been formed from felspars which contain little or no iron, and the -coarser kinds of clay from others containing a greater proportion of -foreign substances. - -Mica, which is used for making lamp chimneys, is a potassium aluminium -silicate. Asbestos, meerschaum, beryl, garnet, jade, and hornblende are -all silicates of various metals. - -Glass is a complex mixture of insoluble silicates with excess of silica. -The varieties in common use are soda glass, Bohemian glass, and lead -glass (which is also called flint glass). Soda glass is mainly a mixture -of calcium and sodium silicates, and is distinguished by its low melting -point, which makes it easy to work at moderate temperatures. It appears -in commerce as plate glass, window glass, and common bottles. Bohemian -glass contains calcium and potassium silicates, and has a high melting -point. It is used for making chemical apparatus. Lead or flint glass -contains the silicates of lead and potassium; this is a dense glass, but -at the same time rather soft. It takes a high polish and is used for -making ornamental or cut-glass ware. - -Remembering that glass is composed of the salts of silicic acid, the -reader will readily understand that the mixture from which it is made -must contain acidic and basic constituents. The acidic or acid-forming -material is in every case silica or sand. This must be pure, and for all -but the commonest kind of bottle or window glass, it must be free from -iron, otherwise the glass will have a more or less pronounced greenish -colour. It must also be fine and even grained. Formerly, the glass sands -used in this country came from Holland and Belgium, but now supplies -from several British sources are being successfully used. - -The basic portion of the glass mixture differs according to the kind of -glass required. An average mixture for soda glass contains sand, 20 -parts; salt cake (sodium sulphate), 10 parts; quicklime, 5 parts; -charcoal, 1 part. For Bohemian glass, pearl ash (potassium carbonate) -takes the place of salt cake, and no charcoal is necessary because the -materials used are finer. For lead glass, the mixture is still further -modified by the use of litharge, or more often red lead, in place of -lime. - -The ingredients are well mixed and thoroughly dried. Waste glass from a -previous batch is also added. The mixture is heated to about 1200° C. in -large pots made of Stourbridge clay, and the heating is continued for as -much as sixteen hours, and until the whole of the material in the pot is -molten and fairly mobile. Scum or glass-gall is removed, and when gas -bubbles have disappeared, the temperature is allowed to fall to -700°-800°, when the glass becomes sufficiently viscous for subsequent -working. The semi-fluid mass is then blown, moulded, or drawn, according -to the kind of article that is required. - -The physical properties of glass will now be considered in order that we -may be able to account for its extended use. Such an inquiry as this, -especially in the case of materials in common use, is often interesting, -because it frequently happens that the special property upon which we -set so much value is an abnormal one and, consequently, the feature -which we take for granted is precisely the one into which we should -inquire most closely. - -The most striking feature of glass is its transparency. This property is -abnormal, if glass is a solid. Consider what happens in most cases. A -substance like nitre melts easily and in the molten state is perfectly -transparent; when it cools, crystals form and, though these individually -may be transparent, yet the solid mass is opaque. The reason for this is -that the solid is not optically homogeneous, and therefore a ray of -light cannot pass through it in a straight line. At each facet of a -crystal light is deviated and reflected, and in the end is almost wholly -scattered. Consequently, an object, even if it can be seen at all, can -be discerned only in a blurred and indistinct fashion through such a -medium. - -There are very good reasons, however, for supposing that glass is not a -true solid but an extremely viscous liquid. If glass is heated, it -softens and begins to flow very sluggishly at first, but afterwards more -readily. There is no abrupt change, as there generally is in passing -from the solid to the liquid state. Similarly in cooling, there is no -point at which it is possible to say that the glass is solidifying. The -view that this substance is really a liquid is perhaps a little -startling at first, but it becomes less so when we observe that a long -glass rod supported at its ends in a horizontal position sags in the -middle and is permanently deformed. - -To avoid that change which would be technically called solidification by -a scientist, the article which has been fashioned in glass is cooled -down very slowly and gradually. This part of the process is called -annealing; it may occupy some days in extreme cases, and it points to -the fact that experience has shown that it is necessary to guard against -some change which would normally take place if this precaution were -neglected. - -The change in glass which annealing is intended to prevent is known as -devitrification. In spite of all precautions, this does occur sometimes, -and specimens of old window glass are often seen to have lost their -transparency completely and to have an opalescent sheen. In these cases, -the silicates have crystallized. - -An extreme case of badly annealed glass is illustrated by Rupert's -drops, a scientific curiosity of very old standing. These are "tears" of -glass made by dropping the molten substance into water. When the tail of -the drop is nipped off, the whole thing is shattered to powder with -something like explosive violence. Clearly there is a very great -internal strain, due to the fact that the outer parts have solidified -and contracted, while the inner part is still warm and dilated. - -Another remarkable feature of glass is the ease and simplicity with -which it can be fashioned into articles of various shapes. As a plastic -material, molten glass almost ranks with clay. This again is due to the -property of passing through a viscous state, that is, one which is -intermediate between a solid and a liquid. - -Water Glass, or soluble glass, is mainly sodium silicate. It is made by -fusing sand or powdered flint with caustic or with mild soda; sometimes, -by digesting crushed flint or chert with caustic soda solution under -considerable pressure in autoclaves or specially constructed boilers. In -the latter case, no extraction is necessary; but in the former, the -residue is treated with water and the solution evaporated until it -becomes a viscous transparent liquid. - -This liquid is used in various ways in industry. It is added to the -cheaper varieties of yellow soap, and is employed as a mordant in dyeing -and printing calico. An artificial sandstone is made by mixing sand, -calcium chloride, and sodium silicate; the two last-named substances -interact to form calcium silicate, which is insoluble in water. For -domestic purposes, water glass is best known in connection with the -preserving of eggs. When the film of water glass dries on the surface of -the egg shell, the latter becomes impervious to air. - - - - - CHAPTER VII - ORGANIC ACIDS - - -Organic Chemistry. About a century ago, when the science of Chemistry -was still in its infancy, several substances were known which could then -only be obtained from animals or plants. The composition of these -substances was not understood, and they were not classified; moreover, -since none of them had ever been prepared artificially, it was supposed -that it was impossible to do this--the reason given was that "vital -force" was necessary for their production. In time, however, some of the -most typical animal and vegetable products were prepared in the -laboratory, and the belief in vital force disappeared. - -In later times it was proved that substances like sugar, starch, urea, -indigo, and a great many more, all contain the element carbon. At the -present time, more than 100,000 compounds of this element are known; and -since they resemble one another, and at the same time differ in several -important respects from the compounds of other elements, it is both -natural and convenient that they should be classed together and studied -separately. This branch of Chemistry is called organic. It must not, -however, be supposed that all organic compounds are necessarily produced -by some living organism. A great many are, but there are many more which -are purely synthetic products. - -Inorganic Chemistry includes all the other elements and their -derivatives. The _element_ carbon, and also some of its simpler -compounds, such as carbon monoxide, carbon dioxide, carbonic acid, and -carbonates, are more appropriately placed in the inorganic section. - -The acids which have been considered up to this point are all inorganic -acids, and those which follow are organic. Sulphuric, nitric, and -hydrochloric acids are often distinguished as the mineral acids in -contradistinction to oxalic, citric, tartaric, and some others which -were first obtained from unripe fruits and therefore called vegetable -acids. - -Organic acids have all the general properties of the class, but they are -much weaker than the mineral acids mentioned above. This is shown by -their solvent action on metals, oxides, and carbonates, which is in all -cases slight. - -Vinegar is the trade name for what is essentially a dilute solution of -acetic acid which has been made by the acetous fermentation of -saccharine fluids containing weak alcohol. In addition to acetic acid, -vinegar contains minute quantities of a large number of compounds. Some -of these help to produce that agreeable flavour and aroma which -distinguishes vinegar from diluted acetic acid. The nature and quantity -of the flavouring constituents depend mainly upon the nature of the -alcoholic solution used. - -Since the acetic acid in vinegar is always produced by fermentation, all -processes for the manufacture of vinegar are essentially arrangements -for promoting the vigorous growth and development of _Mycoderma aceti_, -the organism which produces the vinegar ferment. - -Like all other plants, _Mycoderma aceti_ will flourish only under -certain favourable conditions. In the first place, it requires -nourishment, and therefore certain nitrogen compounds and salts must be -present in the alcoholic solution. These are contained in wines, beer, -cider, and malt liquors, but not in spirits of wine, which is pure -alcohol distilled from liquids which have undergone vinous fermentation. -If the plant is placed in dilute spirits of wine, only a very little -acetic acid is formed, and then the action ceases because the solution -does not contain the necessary food substances. Temperature also has a -very marked effect on growth, the most favourable range being between -68° and 95° F. - -Alcohol is changed to acetic acid by the process of oxidation, and -therefore, in making vinegar, arrangements have to be made to bring -together weak alcohol and air in the presence of the plant. The ferment -which is secreted by the plant then causes an acceleration of the -reaction. There is a considerable amount of similarity between -fermentation and contact action. In this connection, it is interesting -to note that the conversion of alcohol into acetic acid can also be -brought about by exposing a mixture of alcohol vapour and air to the -action of platinum black; in fact, there is one process for making -vinegar in this way. - -French Vinegar. New wine, especially that which contains a low -percentage of alcohol, is liable to many kinds of "sickness." It may -turn bitter, it may turn sour, or it may undergo what is called lactic -fermentation. In either case, it becomes unsaleable as a beverage. Wine -which has turned sour is the best material for making vinegar, and when -this is done by the French or slow process, a product with a very fine -_bouquet_ is obtained. - -The methods adopted are very simple. Formerly, the wine was poured into -barrels leaving the top portion empty, and providing for a current of -air over the surface. The barrels were often set up in rows in the open -air in an enclosure which was then known as a "vinegar field." The -process of souring which had already begun went on naturally, and in the -course of a few months, nearly the whole of the alcohol was converted -into acetic acid. - -The process now in use in some of the French factories is somewhat -similar. Large casks holding about 100 gallons are set up in a room, and -provision is made for keeping the temperature uniform. Each cask is -first acidulated by allowing strong vinegar to stand in it until the -vinegar plant has developed on the surface. The casks are then filled up -very gradually by adding a few gallons of wine every eight or ten days. -When the cask is full, a fraction of the contents is drawn off and -replaced by wine. The process then becomes continuous, until it is -necessary to clean out the generator and start again. - -In recent times, the manufacture of wine vinegar has been carried out on -more scientific principles. The vinegar plant is actually cultivated and -examined microscopically before being used, in order to make sure of the -absence of moulds and bacteria, which set up other fermentations, -producing substances which affect adversely the taste and aroma of the -finished product. The cultivated ferment is then added to the wine in -shallow vessels and the process is carried on as described above. - -Malt Vinegar. A dilute solution of alcohol which is made from malt by -fermentation with yeast contains the nutritive substances necessary for -the growth of the vinegar plant, and can therefore be used as a -starting-point for the manufacture of vinegar. Sprouted barley or malt -is mixed with oats, barley, rice, or other starch-containing material. -The mixture is mashed with warm water and then fermented with yeast, -giving what is called "raw spirit." This is converted into vinegar by -the "quick" process. - -The vinegar generator (Fig. 10) is a large barrel divided into three -compartments by two perforated partitions. The lower disc is fixed about -one-third of the way up the barrel, and near it holes are bored to admit -air. The upper disc, fixed near the top of the barrel, is perforated -with a large number of small holes which are partially stopped up with -short threads or wicks, which hang from the under side. The space -between the two discs is packed with specially prepared beech shavings, -which have been left to stand in strong vinegar until they are covered -with the vinegar plant. - - [Illustration: Fig. 10. QUICK VINEGAR PROCESS] - -The weak spirit is delivered into the upper portion of the barrel and is -distributed in very small drops by the threads; it then passes slowly -over the vinegar plant, to which the air also has free access. When it -reaches the bottom, it overflows into a reservoir and is again passed -through the generator; this is repeated until the product contains the -desired amount of acetic acid. - -The principle of the quick vinegar process is the same as that employed -in making wine vinegar. The speed of the reaction is, however, greatly -increased by having the ferment spread over a very large surface and by -the free circulation of air. It is possible to make wine vinegar by the -quick process, but it is not done, because the product is inferior in -taste and aroma to that made by the slow process. - -Both wine vinegar and malt vinegar when freshly prepared have a -stupefying and unpleasant odour. Before the product is ready for the -market, it has to be matured in barrels. During this process, a small -quantity of alcohol which still remains in the vinegar combines slowly -with some of the acetic acid, producing acetic ester, a substance which -has a pleasant fruity odour. - -The colour of wine vinegar is natural, but vinegar which is produced by -the quick process is colourless or only faintly coloured. Since the -public has a preference for vinegar which is brown in colour, the -product of the quick process is coloured artificially, either by adding -caramel or by preparing the weak spirit from malt which has been -slightly charred in drying. - -Industrial Acetic Acid. The solutions of acetic acid dealt with above -would be too dilute for any industrial purpose; moreover, the acid can -be obtained much more cheaply by the distillation of wood. When wood is -subjected to a high temperature, it is converted into charcoal and, at -the same time, an inflammable gas, an acid liquid, and tar are given -off, and can be collected in suitable vessels. The following table, on -page 73, gives the relative amounts of the various substances obtained -from wood by dry distillation. The quantities are those derived from one -cord, that is, 125 cu. ft. - - _Charcoal _Alcohol _Calcium _Tar in _Wood oil _Turpentine - in in acetate gallons._ in gallons._ - bushels._ gallons._ in lbs._ gallons._ - Hard 40-50 8-12 150-200 8-20 - woods - Resinous 25-40 2-4 50-100 30-60 30-60 Heavy woods - woods 12-25 - Light woods - 2-10 - Sawdust 25-35 2-4 45-75 - -The aqueous liquid that distils over contains methyl alcohol (wood -spirit), acetone, and acetic acid. The crude mixture is known as -pyroligneous acid. This is neutralized with milk of lime or soda ash, -which converts acetic acid into calcium or sodium acetate, but has no -action on the methyl alcohol and acetone which are also present. The -mixture is then distilled, when methyl alcohol, acetone, and water pass -over into the distillate, leaving the acetate in the retort. - -To obtain the free acid from the acetate, the latter is well dried and -then distilled with concentrated sulphuric acid. Acetic acid, being the -more volatile of the two acids, distils over, and is nearly pure. - -The method of removing the last traces of water depends upon the fact -that acetic acid solidifies at 17° C. The acid, which is nearly, but not -quite, free from water, is cooled until a portion solidifies. The part -which still remains liquid is poured away, and the process is repeated -until a residue is obtained which solidifies as a whole. This is glacial -acetic acid, so called because it is a mass of glistening plates which -look like newly-formed ice. - - - The Acetates - -Aluminium Acetate, made by dissolving alumina in acetic acid, is the -"red liquor" which is used as a mordant in dyeing. It is a colourless -liquid, but is called "red liquor" because it is used with dyes which -give a red colour. - -Ferrous Acetate, made in a similar way from scrap iron and acetic acid, -is the "black liquor" used in dyeing. - -Verdigris, or basic copper acetate, is a valuable pigment. It is made by -interposing cloths soaked in vinegar between plates of copper. After the -action has been allowed to go on for a long time, the plates are washed -with water and the verdigris is scraped off. The finest verdigris is -made in France in the wine-producing district around Montpellier. Here, -instead of cloths soaked in vinegar, the solid residue from the wine -presses is spread in layers between the copper plates. The product made -in this way is called _vert de Montpellier_. - - [Illustration: Fig. 11. DUTCH PROCESS FOR WHITE LEAD] - -Verdigris, like all the copper compounds, is extremely poisonous. It is -very liable to be formed on the surface of copper utensils used for -cooking purposes. - -Lead Acetate, or sugar of lead, is used in large quantities in the -colour industry for making various reds and yellows. It is prepared by -dissolving the metal or its oxide (litharge) in acetic acid. - -The slow action which acetic acid vapour has upon the metal lead finds a -very interesting application in what is known as the Dutch process for -the manufacture of white lead[4] for paint. The metal is cast into grids -or spirals, which are placed on the shoulders of the specially made pots -sketched in Fig. 11. A little dilute acetic acid is poured into each of -the pots, which are then arranged side by side on a thick layer of tan -bark, stable manure, or other material which will heat by fermentation. -The first layer of pots is then boarded over; another layer of pots is -placed upon this, and so on, tier upon tier, until the shed is quite -full. The heat developed by the fermenting material vaporizes the acetic -acid, and this vapour corrodes the lead, forming basic lead acetate. The -carbon dioxide which is also produced during fermentation converts the -acetate into the carbonate, which falls as a heavy white powder into the -pots. - -Future Supply of Acetic Acid. When all the operations involved in the -production of acetic acid from wood, from the felling of the tree to the -final separation of the glacial substance, are taken into consideration, -it will be readily understood how it is that this acid has never been -cheap when compared with other acids used on an equally large scale. In -addition to this, the competition for wood for paper-making and for the -very numerous cellulose industries is rapidly increasing. It is, -therefore, not surprising to learn that chemists have turned their -attention towards the discovery of newer and cheaper methods of making -acetic acid. - -Such a process seems to have been worked out in Germany. The -starting-point is acetylene gas made by the action of water on calcium -carbide. When this gas is passed through sulphuric acid containing -suspended mercuric oxide or dissolved mercury salt, the acetylene is -oxidized first to aldehyde and then to acetic acid. - -If this process should prove to be successful, it will form the -starting-point of a new and important industry, for, apart from the -large amount of acetic acid which is used in commerce, there is the -production of the very important solvent known as acetone, which can be -made from acetic acid by a very simple operation. - -Tartaric Acid. Grape juice contains a large quantity of potassium -hydrogen tartrate dissolved in it; when the liquid is fermented and -alcohol is formed, this salt crystallizes out because it is not soluble -in alcohol. After the new wine has been poured off, the salt is found as -a brownish crystalline residue adhering to the sides of the vat. Also -the salt goes on crystallizing after the wine is put into barrels, and -forms an incrustation on the sides. This is called the _lees_ or -sediment of wine. In commerce, the substance is known as _argol_ -(sometimes spelt _argal_), and also _tartar_ of wine. - -Crude argol is purified by dissolving it in water and destroying the -colour by boiling with animal charcoal. When the clear liquid obtained -from this mixture by filtration is evaporated, a white crystalline -substance separates out. This is potassium hydrogen tartrate or _cream -of tartar_. - -Tartaric acid is obtained from cream of tartar. The salt is dissolved in -water and nearly neutralized with milk of lime. Insoluble calcium -tartrate is precipitated, and potassium tartrate remains in solution. A -further quantity of calcium tartrate is obtained by adding calcium -chloride to the solution just mentioned. The two precipitates of calcium -tartrate are then mixed and decomposed by dilute sulphuric acid, and -after the calcium sulphate is filtered off, tartaric acid is obtained as -a solid by evaporating the clear liquid. - -The general properties of tartaric acid are well known. It is soluble in -water, giving a solution which has a pleasantly acid taste. - -Citric Acid. The sharp flavour of many unripe fruits is due to the -presence of citric acid; the juice of lemons contains 5-6 per cent. of -the acid. The free acid is obtained in a manner precisely similar in -principle to that described for tartaric acid. - -Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely -distributed in the vegetable kingdom. These compounds are present in -wood sorrel (_Oxalis acetosella_), in rhubarb, in dock, and in many -other plants. The acid is made on a large scale by mixing pine sawdust -to a stiff paste with a solution containing caustic soda and potash. The -paste is spread out on iron plates and heated, care being taken not to -heat the mixture to the point at which it chars. The mass is then -allowed to cool, and is mixed with a small quantity of water to dissolve -out the excess of alkali. This is recovered and used again. - -Sodium oxalate, which is the main product of the reaction described -above, is dissolved in water and treated with milk of lime, whereby -insoluble calcium oxalate is obtained, which is subsequently decomposed -with sulphuric acid, yielding oxalic acid. - -Potassium hydrogen oxalate is sometimes called _salts of sorrel_, and -potassium quadroxalate, _salts of lemon_. The most familiar use of the -latter substance is in the removal of ink stains. - -Oxalic acid and its salts are poisonous. The free acid has sometimes -been mistaken for sugar with fatal results. - -Formic Acid (_L. formica_, an ant) is found both in the vegetable and in -the animal kingdom. If the leaf of a stinging nettle is examined with a -microscope, it is seen to be covered with long pointed hairs having a -gland at the base. This gland contains formic acid. When the nettle is -touched lightly, the fine point of the hair punctures the skin, and a -subcutaneous injection of formic acid is made, which quickly raises a -blister. - -The inconvenience which arises from the stings of bees and wasps, also -from the fluid ejected by ants when irritated, is due to formic acid. -The remedy in each case is the same; the acid must be neutralized as -quickly as possible with mild alkali, such as washing soda. - -Formic acid was first made by distilling an infusion of red ants. It is -now made from glycerine and oxalic acid. - -The Fatty Acids. Animal fats and vegetable oils are similarly -constituted bodies. They are composed mainly of three chemical compounds -known as stearin, palmitin, and oleïn. Of these, stearin and palmitin -are solids at ordinary temperatures, while oleïn is a liquid. Hard fats -like those of mutton and beef are composed mainly of stearin; fats of -medium hardness contain stearin, palmitin, and some oleïn; while oils -such as cod-liver oil and olive oil are nearly pure oleïn. - -Stearin, palmitin, and oleïn are analogous in composition to salts. -Their proximate constituents are glycerine and certain organic acids, -stearic, palmitic, and oleïc respectively. - -In order to obtain the fat free from tissue which it contains in its -natural state, it is tied up in a muslin bag and heated in boiling -water. The fat is squeezed out through the meshes of the fabric and -floats on the surface of the water as an oil which solidifies on -cooling. This clarified fat is called tallow. - -All fats and vegetable oils can be resolved into their two constituents, -the acid and the glycerine. This can be brought about by heating the fat -with water to about 200° C. This operation must be carried out in a -vessel capable of withstanding pressure and closed with a safety valve; -otherwise, the requisite temperature could not be obtained. After this -treatment, there is left in the vessel an oily layer which solidifies on -cooling and an aqueous layer which contains the glycerine. The -solidified oily layer is the fatty acid. In the case of mutton or beef -tallow, it would be mainly a mixture of stearic and palmitic acids. This -mixture is used to make "stearin" candles. The acids themselves are -wax-like solids without any distinctive taste. Stearic acid melts at 69° -C. and palmitic at 62° C. They have no perceptible action on the colour -of litmus, neither have they any solvent action on metals or carbonates. -We should not recognize these substances as acids at all were it not for -the fact that they combine with alkalis, forming salts. - -The salts of the fatty acids are called soaps. To make soap, the fat is -boiled with caustic alkali or caustic lye, as it is more often called. -This breaks the fat up primarily into the acid and glycerine; but in -this case, instead of obtaining the acid as the final product as we did -above by heating with water under pressure, we get the sodium or -potassium salt of the acid according to the alkali used. When caustic -soda is used, the product is a hard soap; when caustic potash is used, -it is a soft soap. The treatment of fats in this way with caustic -alkalis is called "saponification." - - - - - CHAPTER VIII - MILD ALKALI - - -Caustic and Mild. There are two classes of alkalis distinguished by the -terms caustic and mild. If a piece of all-wool material is boiled with a -solution of caustic soda or potash, it dissolves completely, giving a -yellow solution. Mild alkali will not dissolve flannel, though it may -have some slight chemical action causing shrinkage. Partly for this -reason, and partly because commercial washing soda often contains a -little caustic soda, woollen garments must not be boiled or even washed -in hot soda water. - -The disintegrating action of the caustic alkalis is also illustrated by -the use of caustic soda in the preparation of wood pulp for paper -making. Tree trunks are first torn up and shredded by machinery; but -notwithstanding the power of modern machinery, the fibre is not nearly -fine enough for the purpose until it has been "beaten" with a solution -of caustic soda, whereby the pulp is brought to a smooth and uniform -consistency like that of thin cream. - -Mild Soda and Potash. Until the middle of the eighteenth century, it was -thought that the soluble matter extracted from the ashes of all plants -was the same. In 1752 it was shown that the substance obtained in this -way from plants which grew in or near the sea differed from that from -land vegetation by producing a golden yellow colour when introduced into -the non-luminous flame of a spirit lamp, while that from land plants -gave to the flame a pale lilac tinge. The former substance is now known -as mild soda, and the latter as mild potash. - -At this point it is well to make it clear to the reader that there are -two bodies commonly called soda, and two called potash. One of each pair -is caustic and one mild. - -By a simple chemical test it is easy to distinguish a mild from a -caustic alkali. When a little dilute acid is added to the former, there -is a vigorous effervescence caused by the escape of carbon dioxide, but -no gas is given off when a caustic alkali is treated in the same way. -The liberation of carbon dioxide on the addition of acids shows that the -mild alkalis are carbonates. - -Washing Soda is so well known, that very little description of its -external characteristics is necessary. It is a crystalline substance, -easily soluble in water. The crystals, when freshly prepared, are -semi-transparent; but after exposure to air for some time, they are -found to lose their transparency and to become coated with an opaque -white solid which crumbles easily. This change in appearance is -accompanied by a loss in weight. - -Crystals of soda melt very easily on the application of heat and, on -continued heating, the liquid seems to boil. When this operation is -carried out in a vessel attached to a condenser, the vapour that is -given off from the melted soda condenses to a clear colourless liquid -which, on examination, proves to be water. When no more water collects -in the receiver, the vessel contains a dry, white solid, which by any -chemical test that may be applied is shown to be the same as washing -soda, but it contains no water of crystallization and has a different -crystalline form. This substance is anhydrous sodium carbonate, or soda -ash as it is called in commerce. When soda ash is mixed with water, it -combines with about twice its own weight of that liquid, forming soda -crystals again. - -Washing soda, then, contains nearly two-thirds of its weight of water. -Some of this water is given off spontaneously when the soda is exposed -to air; the water may even be said to evaporate. This accounts for the -loss of weight observed and also for the formation of the white layer of -partially dehydrated soda over the surface of the crystal. The property -of losing water in this way is common to most crystals containing a high -percentage of water of crystallization. The phenomenon is known as -"efflorescence." It may here be observed that crystals of washing soda -which have become coated over in this way contain relatively more soda -than those which are transparent. - -Natural Soda. In Egypt, Thibet, and Utah, there are tracts of country -where the soil is so impregnated with soda that the land is desert. The -separation of the soda from the earth is a simple operation, for it is -only necessary to agitate the soil with water and, after the insoluble -matter has settled down, to evaporate the clear solution until the soda -crystallizes out. - -In addition to alkali deserts, there are also alkali lakes. Those in -Egypt are small, nevertheless, about 30,000 tons of soda per annum are -exported from Alexandria. Owens Lake in California is said to contain -sufficient soda to supply the needs of North America; while in the East -African Protectorate, beneath the shallow waters of Lake Magadi -(discovered in 1910), there is a deposit of soda estimated at -200,000,000 tons. - -The Leblanc Process. At the present time, the greater part of the -world's supply of soda is made from common salt by two processes. The -older of these, which is known as the Leblanc process, was introduced in -France towards the end of the eighteenth century. In those days soda was -very dear, for the main supply came from the ashes of seaweeds; -wherefore the French Academy of Sciences, in 1775, offered a prize for -the most suitable method of converting salt into soda on a manufacturing -scale. The prize was won by Nicholas Leblanc, who in 1791 started the -first soda factory near Paris. These were the days of the French -Revolution; the "Comité de Sûreté Général" abolished monopolies and -ordered citizen Leblanc to publish the details of his process. - - [Illustration: Fig. 12. SALT CAKE FURNACE] - -The first alkali works were established in Great Britain in 1814. The -total amount of soda now made in this country every year is about -1,000,000 tons, of which nearly one-half is still made by the Leblanc -process. - -Salt Cake. The first stage of the Leblanc process consists in mixing a -charge of salt weighing some hundredweights with the requisite amount of -"chamber" sulphuric acid. The operation is carried out in a circular -cast-iron pan (D, Fig. 12) about 9 ft. in diameter and 2 ft. deep. The -pan is covered over with a dome of brickwork, leaving a central flue (E) -for the escape of hydrochloric acid gas which is produced. At first, the -reaction takes place without the application of heat, but towards the -end the mass is heated for about one hour. The contents of the pan are -then raked out on to the hearth of a reverberatory furnace (_a_, _b_) -and more strongly heated. More hydrochloric acid gas is given off, and -the reaction is completed. The solid product which remains is impure -Glauber's salt (sodium sulphate), and is known in the trade as "salt -cake." - -Black Ash. In the second stage of the Leblanc process, salt cake is -converted into black ash. The salt cake is crushed and mixed with an -equal weight of powdered limestone or chalk and half its weight of coal -dust. This mixture is introduced into a reverberatory furnace (Fig. 13) -by the hopper K, and heated to about 1000° C. by flames and hot gases -from a fire at _a_. During this operation, the mass is kept well mixed, -and after some time it is transferred to _h_ where the temperature is -higher. The mixture then becomes semi-fluid and carbon monoxide gas is -given off. - - [Illustration: Fig. 13. BLACK ASH FURNACE] - -The formation of carbon monoxide within the semi-solid mass renders it -porous. This is an advantage, because it greatly facilitates the -subsequent operation of dissolving out the soluble sodium carbonate. The -appearance of the flames of carbon monoxide at the surface of the black -ash indicates the end of the process. The product is then worked up into -balls and removed from the furnace. - -The chemical changes which take place in making black ash are probably -as follows: Carbon (coal dust) removes oxygen from sodium sulphate, -which is thus changed to sodium sulphide. This substance then reacts -with the limestone (calcium carbonate), forming sodium carbonate (soda) -and calcium sulphide. - -Extraction of Soda. It now only remains to dissolve out the soda from -the insoluble impurities with which it is mixed in the black ash. It is -evident that the smaller the amount of water used for this purpose the -better, because the water has subsequently to be got rid of by -evaporation. The process of extraction is, therefore, carried out -systematically. The black ash is treated with water in a series of tanks -which are fitted with perforated false bottoms. The soda solution, which -is heavier than water, tends to sink to the bottom and, after passing -through the perforations, is carried away by a pipe to the second tank, -and so on throughout the series. The fresh water is brought first into -contact with the black ash from which nearly all the soda has been -extracted. - -The method of finishing off the black ash liquor differs according to -the final product which the manufacturer desires to obtain, for the -liquor contains caustic soda as well as mild soda. For the present, we -will suppose that the end product is to be washing soda. In this case, -carbon dioxide is passed into the liquor to convert what caustic soda -there is into mild soda. - -The clarified soda liquor is then evaporated until crystals of soda -separate out. The first part of this process is carried out in large -shallow pans (P. Fig. 13), using the waste heat of the black ash -furnace, and finally in vats containing steam-heated coils. As the -crystals separate out, they are removed, drained, and dried. - -Alkali Waste. Black ash contains less than half its weight of soda, so -that for every ton of soda produced there is from a ton and a half to -two tons of an insoluble residue which collects in the lixiviating and -settling tanks. This residue is known as alkali waste. - -Alkali waste is of no particular value. It is not even suitable as a -dressing for the land, and since it is not soluble in water there is no -convenient means of disposing of it. Consequently, it is just -accumulated at the works and, as the heap grows at an alarming rate, it -cumbers much valuable ground. Moreover, it contains sulphides from -which, under the influence of air and moisture, sulphuretted hydrogen is -liberated. Alkali waste, therefore, has a very unpleasant odour. - -The whole of the sulphur which was contained in the sulphuric acid used -in the first stage of the process remains in the alkali waste, mainly as -calcium sulphide. A plant for the recovery of this sulphur is -established in some of the larger works. The alkali waste is mixed with -water to the consistency of a thin cream, in tall, vertical cylinders. -Carbon dioxide under pressure is forced into the mixture, and this -converts the calcium sulphide into calcium carbonate and sets free -hydrogen sulphide, which, when burnt with a limited supply of air, -yields sulphur. - -By this process, the most unpleasant feature of alkali waste, namely, -the smell, is removed. The calcium carbonate which remains is of very -little value. Some of it is used in making up fresh charges for the -black ash process and some for preparing Portland cement, for which -finely-ground calcium carbonate is required; the remainder is thrown on -a heap. - -Bicarbonate of Soda. Bicarbonate of soda can be easily distinguished -from washing soda. It is a fine, white powder similar in appearance to -the efflorescence on soda crystals. It does not contain any water of -crystallization. - -When bicarbonate of soda is heated, it does not melt, and, as far as its -external appearance is concerned, it does not seem to undergo any -change. If, however, suitable arrangements are made, water and carbon -dioxide gas can be collected, and the sodium bicarbonate will be found -to have lost 36·9 per cent. of its weight. The substance which remains -is identical with that obtained by heating soda crystals, that is, -anhydrous sodium carbonate. Sodium bicarbonate is, therefore, a compound -of sodium carbonate and carbonic acid. - -The most familiar use of this compound is indicated by its common names -"baking-soda" and "bread-soda." It is mixed with dough or other similar -material in order to keep this from settling down to a hard solid mass -in baking. The way in which bicarbonate of soda prevents this will be -readily understood when it is remembered that an ounce of this substance -liberates more than 2,300 cu. in. of carbon dioxide when it is heated. -When the bicarbonate of soda is well mixed with the ingredients of the -cake or loaf and disseminated throughout the mass, each particle will -furnish (let us say) its bubble of gas. Since these cannot escape, a -honey-combed structure is produced. - - [Illustration: Fig. 14. THE SOLVAY PROCESS] - -Baking powder is a mixture of bicarbonate of soda and ground rice; the -latter substance is merely a solid diluent. - -The Solvay Process. Soda ash is one of the principal forms of mild -alkali used in commerce. Large quantities of this substance are made by -heating bicarbonate of soda. We shall now consider another alkali -process in which this substance is the primary product. - -For the greater part of the first century of its existence, the Leblanc -soda process had no rival, although another method, known as the -ammonia-soda process, was patented as early as 1838. In this case, -however, as in many others, expectations based on the experiments -carried out in the laboratory were not realized when the method came to -be tried under manufacturing conditions. It was not until 1872 that -Ernest Solvay, a Belgian chemist, had so far solved the difficulties, -that a new start could be made. In that year, about 3,000 tons of soda -were produced by the ammonia-soda or Solvay process, as it has now come -to be known. Since then, however, the quantity produced annually has -been steadily increasing, until at the present time it amounts to more -than half of the world's supply. - -The Solvay process is very simple in theory. Purified brine is saturated -first with ammonia gas and then with carbon dioxide. Water, ammonia, and -carbon dioxide combine, forming ammonium bicarbonate, which reacts with -salt (sodium chloride), producing sodium bicarbonate and ammonium -chloride. - -The principal reaction is carried out in a tower (Fig. 14 (1), _a_, _a_) -from 50 to 65 ft. in height and about 6 ft. in diameter. At intervals of -about 3-1/2 ft. throughout its length, the tower is divided into -sections by pairs of transverse discs, one flat with a large central -hole, and one hemispherical and perforated with small holes (Fig. 14 -(2)). The discs are kept in position by a guide rod G. Fig. 14 (3) shows -a better arrangement of the guide rods. In modern works, the space -between the discs is kept cool by pipes conveying running water. The -ammoniated brine is led into the tower near its middle point. The carbon -dioxide is forced in at E in the lowest segment, and as it passes up the -tower it is broken up into small bubbles by the sieve plates. Sodium -bicarbonate separates out as a fine powder, which makes its way to the -bottom of the tower suspended in the liquid. - -The perforated plates are necessary for the proper distribution of -carbon dioxide through the brine. They are, however, a source of -trouble, because the holes quickly become blocked up with sodium -bicarbonate, and every ten days or so it is necessary to empty the tower -and clean it out with steam or boiling water. - -Recovery of Ammonia. The production of 1 ton of soda ash by the Solvay -process involves the use of a quantity of ammonia which costs about -eight times as much as the price realized by selling the soda. It is -evident that the success of the process as a commercial venture depends -largely on the completeness with which the ammonia can be recovered. - -During the process, ammonia is converted into ammonium chloride, which -remains dissolved in the residual liquor. From this ammonia gas is set -free by adding quicklime and by blowing steam through the mixture. It is -now claimed that 99 per cent. of the ammonia used in one operation is -recovered. - -Soda Ash. The bicarbonate of soda produced by the Solvay process is -moderately pure. For all ordinary purposes, it is only necessary to wash -it with cold water to remove unchanged salt, and after drying, it is -ready to be placed on the market if it is to be sold as bicarbonate. The -greater part of the Solvay product, however, is converted into soda ash -by the application of heat. If soda crystals are required, the soda ash -is dissolved in water and crystallized. - -In many ways, the Solvay process compares very favourably with the older -method. It is an advantage to start with brine, for that is the form in -which salt is very often raised from the mines. The end product is -relatively pure; moreover, it is quite free from caustic soda, which for -some purposes for which soda ash is used is a great recommendation. -There is no unpleasant smelling alkali waste. On the other hand, the -efficiency of the Solvay process is not high, for only about one-third -of the salt used is converted into soda. This would make the process -impossible from the commercial point of view were it not for the -cheapness of salt. - -The Leblanc process, too, has its advantages. In the next chapter we -shall see that it is adaptable for the production of caustic as well as -mild alkali. The chlorine which is recovered in the Leblanc process is a -very valuable by-product. In the Solvay process, chlorine is lost, for -hitherto no practicable method has been found for its recovery from -calcium chloride. - -The position with regard to the future supply of alkali is very -interesting. The competition between the Leblanc and the Solvay -processes for supremacy in the market is very keen. At the same time, -both processes are in some degree of danger of being supplanted by the -newer electrical methods, which will be mentioned in the last chapter. - -The following table shows very clearly the rapid progress made by the -Solvay process in ten years. The quantities are given in _tonnes_ (1 -tonne = 0·9842 ton). - - 1884. 1894. - _Leblanc _Solvay _Leblanc _Solvay - soda._ soda._ soda._ soda._ - Great Britain 380,000 52,000 340,000 181,000 - Germany 56,500 44,000 40,000 210,000 - France 70,000 57,000 20,000 150,000 - United States -- 1,100 20,000 80,000 - Austria-Hungary 39,000 1,000 20,000 75,000 - Russia -- -- 10,000 50,000 - Belgium -- 8,000 6,000 30,000 - 545,500 163,100 456,000 776,000 - -Mild Potash. Potassium carbonate (mild potash) was formerly obtained -from wood ashes. The clear aqueous extract was evaporated to dryness in -iron pots, and the substance was on this account called _potashes_; -later, potash. A whiter product was obtained by calcining the residue, -and this was distinguished as _pearl-ash_. Chemically pure potassium -carbonate was formerly obtained by igniting cream of tartar (potassium -hydrogen tartrate) with an equal weight of nitre. It is for this reason -that potassium carbonate is sometimes called "salt of tartar." - -About the middle of last century, natural deposits of potassium chloride -were discovered in Germany. The beds of rock salt near Stassfurt are -covered over with a layer of other salts, and for many years these were -removed and cast aside as "waste salts" (_abraumsalze_). When at a later -date they were examined more carefully, they were found to contain -valuable potassium compounds, notably the chloride. After that -discovery, mild potash was made by the Leblanc process., and Germany -controlled the world's markets for all potassium compounds. - -At the outbreak of war, the German supplies of potassium compounds -ceased as far as the allied nations were concerned, and an older method -of making potassium chloride from _orthoclase_ or potash-felspar was -revived. This involves the heating of the powdered mineral to a high -temperature after mixing it with calcium chloride, lime, and a little -fluorspar. The potassium chloride is then extracted from the fused mass -with water. This method has been worked with great success in America, -and it is claimed that potassium chloride can be made in that country at -a cost which is lower than that formerly paid for the imported article. - -Mild potash and soda are so very similar in chemical properties that in -most cases it is immaterial which compound is used. In all cases in -which there is this choice, soda is employed, both because it is cheaper -and because it is more economical, for 106 parts of soda ash are -equivalent to 138 parts of potash. There are, however, some occasions -when soda cannot be substituted, notably for the manufacture of hard -glass and soft soap, and for the preparation of caustic potash, -potassium dichromate, and other potassium salts. - -Potassium Bicarbonate. This resembles the corresponding sodium salt in -nearly every respect. It is, however, much more readily soluble in -water, so much so, that it is not possible to obtain this substance by -the Solvay method. It is made from potassium carbonate by saturating a -strong aqueous solution of that substance with carbon dioxide. - - - - - CHAPTER IX - CAUSTIC ALKALIS - - -The Alkali Metals. The discovery of current electricity in 1790 -furnished the chemist with a very powerful agency for bringing about the -decomposition of compounds. Hydrogen and oxygen were soon obtained by -passing an electric current through acidulated water; and in 1807, Sir -Humphry Davy, who is perhaps better remembered for his invention of the -miners' lamp, isolated the metals sodium and potassium by subjecting -caustic soda and caustic potash respectively to the action of the -current. - -Sodium and potassium are very remarkable metals. They are only a little -harder than putty, and can easily be cut with a knife or moulded between -the fingers. When exposed to the air, they rust or oxidize very rapidly, -so much so that they have to be preserved in some mineral oil or in -airtight tins. They are lighter than water, which they decompose with -the liberation of hydrogen, and under favourable circumstances the -hydrogen takes fire so that the metals appear to burn on the surface of -the water. After the reaction is over and the sodium or potassium has -disappeared, a clear colourless liquid remains which has a strongly -alkaline reaction, and when this is evaporated until the residue -solidifies on cooling, caustic soda or potash is obtained. For very -special purposes, the caustic alkalis are sometimes made by the action -of the metals on water, but for production on a large scale, less -expensive methods are adopted. - -Caustic Alkali is obtained from the corresponding mild alkali in the -following way. The substance--washing soda, for example--is dissolved in -water and the solution is warmed. Lime is stirred into this solution, -and from time to time a small test portion of the _clear_ supernatant -liquid is removed and mixed with a dilute mineral acid. When this ceases -to cause effervescence, the change is complete. The clear liquid is now -separated from the solid matter (excess of lime together with calcium -carbonate) and evaporated in a metal dish. Since the caustic alkalis are -extremely soluble in water, they do not crystallize as do most of the -compounds previously described. Evaporation is, therefore, carried on -until the liquid which remains solidifies when cold. - -Caustic Soda. To describe the process by which caustic soda is -manufactured, we must return to the making of black ash. The mixture -from which black ash is made contains limestone. It is heated to 1000° -C., which is a sufficiently high temperature to convert limestone into -lime. When the black ash is subsequently treated with water, the lime -which is present converts some of the mild alkali to caustic; -consequently, black ash liquor always contains both alkalis. - -When the manufacturer intends to make caustic soda and not soda -crystals, the composition of the black ash mixture is varied by adding a -larger proportion of limestone, so that there may be an excess of lime -in the black ash produced. The treatment with water is carried out as -described under washing soda, and then more lime is added to convert the -mild soda into caustic soda. After the excess of lime and other -suspended matter has settled down, the clear caustic liquor is -evaporated in iron kettles until it becomes molten caustic, which will -solidify on being allowed to cool. - -There are various grades of caustic soda on the market differing one -from another in purity. The soap manufacturer uses caustic liquor or lye -containing about 40 per cent. of caustic soda. For other purposes, the -solid containing from 60 to 78 per cent. is used. Sometimes the product -is whitened by blowing air through the strong caustic liquor or by the -addition of a little potassium nitrate. Finally, for analytical -purposes, caustic soda is purified by dissolving it in alcohol and -subsequently evaporating the clear liquid. - -Caustic Potash. The methods for the preparation of the corresponding -potassium compound are precisely the same as those described for caustic -soda; in fact, wherever the words sodium and soda occur in this chapter, -the reader can always substitute potassium and potash respectively. - -Caustic Lime. Apart from its use in making mortar and cement, lime is -very often employed to neutralize acids. For this purpose, a suspension -in water, called milk of lime, is generally used, for lime itself is not -very soluble. Probably it is only the soluble part which reacts; -nevertheless, as soon as this is used up, more of the solid dissolves, -and in this way the action goes on as if all the lime were in solution. - -Lime is also a very valuable substance in agriculture, especially on -damp, boggy land, where there is much decaying vegetable matter, and on -land which has been liberally manured. The soil in these cases is very -likely to become acid and is then unproductive. Lime is added to -"sweeten" the soil; in other words, to neutralize the acid. - -Ammonia. The pungent smelling liquid popularly known as "spirits of -hartshorn" is a solution of ammonia gas in water. It is a caustic alkali -and, as such, is sometimes used to remove grease spots. Here, however, -we shall consider ammonia only in connection with ammonium salts, some -of which are used in very large quantity as fertilizers. - -The principal source of ammonia at the present time is the ammoniacal -liquor obtained as a by-product in the manufacture of gas for heating -and lighting. Coal contains about 1 per cent. of nitrogen, and when it -is distilled, some of this nitrogen is given off as ammonia, which -dissolves in the water produced at the same time. This liquid is -condensed in the hydraulic main and in other parts of the plant where -the gas is cooled down. - -Gas liquor contains chiefly the carbonate, sulphide, sulpho-cyanide, and -chloride of ammonia, together with many other substances, some of which -are of a tarry nature. It would not be practicable to evaporate this -liquid with a view to obtaining the ammonium salts, because it is only a -very dilute solution. Hence, after the removal of tar, the liquor is -treated in such a way that ammonia is set free. - -In some cases the liberation of ammonia is accomplished by blowing -superheated steam into the liquor, which sets free the ammonia which is -combined as carbonate, sulphide, and sulpho-cyanide, but not that which -is present as chloride. In other works, the gas liquor is mixed with -milk of lime, which liberates all the combined ammonia. The ammonia is -then expelled from the mixture by a current of steam or air and steam. -In both cases, the gas which is given off is passed into sulphuric acid, -whereby ammonium sulphate is formed in solution and afterwards obtained -as a solid by evaporation. - - - Ammonium Salts - -Ammonium Chloride. Like all other alkalis, ammonia solution neutralizes -acids, forming salts. With hydrochloric acid, it produces the white -solid known as _sal ammoniac_ or ammonium chloride. This compound is -familiar as the one required to make the liquid used in a Leclanché -cell, which is generally used as the current generator for electric -bells. - -Ammonium Carbonate, which is also called stone ammonia and salt of -hartshorn, is made by subliming a mixture containing two parts chalk and -one part ammonium sulphate. It is a white solid which gives off ammonia -slowly and is, therefore, used as the basis for smelling salts. - -Ammonium Nitrate is obtained by passing ammonia gas into nitric acid -until it is neutralized. It is a white solid, which melts easily on -being heated, and breaks up into water and nitrous oxide (laughing gas), -which is the "gas" administered by dentists. Ammonium nitrate is also -used in the composition of some explosives: for example, "ammonite" is -said to contain 80 per cent. of this substance. - -Ammonium Sulphate is used chiefly as an artificial manure; the amount -required for this purpose throughout the world is over 1,500,000 tons -every year. - -Synthetic Ammonia. Though the soluble compounds of nitrogen are fairly -abundant, the supply is by no means equal to the demand, because such -enormous quantities are required for agricultural purposes. It has been -already said that ammonia is obtained as a by-product in the -distillation of coal, and it has been repeatedly pointed out that our -coal supplies are far from inexhaustible; moreover, coal gas may not -always be used for lighting and heating. It, therefore, becomes a very -important question as to how the future supply of ammonium salts is to -be maintained. - -Ammonia is a very simple compound formed from the elements nitrogen and -hydrogen, and, as before mentioned, the supply of free nitrogen in the -air is literally inexhaustible. In recent years, the efforts of chemists -have been directed towards finding a method of converting the free -nitrogen of the air into some simple soluble compound. This problem is -usually spoken of as the "fixation of nitrogen." - -In the Haber process, nitrogen obtained by the fractional distillation -of liquid air is mixed with three times its volume of hydrogen, and this -mixture is heated to between 500°C. and 700°C. under a pressure of 150 -atmospheres (nearly 1 ton to the square inch) and in the presence of a -contact agent. Under these conditions, nitrogen and hydrogen combine to -form ammonia, which is condensed by passing the mixed gases into a -vessel cooled with liquid air, any unchanged nitrogen and hydrogen being -passed back again over the contact substance. - -The problem of making ammonia from the air is closely connected with -that of making nitric acid from the same source. In some experiments the -two are combined, and ammonium nitrate is produced directly. Ammonia -made by the Haber process, or some modification, is mixed with -atmospheric oxygen and passed through platinum gauze heated to low -redness. This results in the formation of nitric oxide, which is further -oxidized by atmospheric oxygen; and finally, from a mixture of oxides of -nitrogen, water vapour, and ammonia, synthetic ammonium nitrate is -obtained. - - - - - CHAPTER X - ELECTROLYTIC METHODS - - -One of the most noteworthy developments of modern chemical industry has -been the increasing use of electricity as an agent for bringing about -changes in matter. This has followed naturally from the reduction in the -cost of electricity, due in great measure to the utilization of natural -sources of energy which for untold ages had been allowed to run to -waste. - -This last achievement is likely to produce such a change in economic -conditions that it is worth while giving a little thought to what may be -called a newly-discovered asset of civilization. One example will make -this clear. In the bed of the Niagara river, which flows from Lake Erie -to Lake Ontario, there is a sudden drop of 167 ft. over which the water -rushes with tremendous force and expends its energy in producing heat -which cannot be utilized. This is a waste of energy, but it cannot be -circumvented because no method has yet been found to control the waters -of the Falls themselves. Nevertheless, by leading the head waters -through suitable channels from the high level to the low, it is possible -to use the energy to drive turbines, which, in their turn, drive dynamos -which produce the current. This is merely the conversion of the energy -of running water into electrical energy; and while the sun remains, this -supply of energy will be forthcoming in undiminished quantity, because -by the heat of the sun the water is lifted again as vapour, which -descends as rain to replenish the sources from which the Niagara flows. - -Electricity is employed in chemical industry in two ways. In the first -place, it may be used to produce very high temperatures required for the -reduction of some metallic ores, for melting highly-refractory -substances, and for making steel. It is, however, rather with the second -method, called electrolysis, that we are here mainly concerned. - - [Illustration: Fig. 15. THE ELECTROLYSIS OF SALT SOLUTION] - -Solutions of acids, bases, and salts, and in some cases the fused -substances themselves, conduct the electric current; but at the same -time they suffer decomposition. This method of decomposing a substance -is known as _electrolysis_, or a breaking up by the agency of -electricity. - -The apparatus required in a very simple case is shown in Fig. 15. It -merely consists of some suitable vessel to contain the liquid; two -plates--one to lead the current into the solution, the other to lead it -away again--and wires to connect the plates to the poles of a battery, -storage-cell, or dynamo. Each plate is called an _electrode_, and -distinguished as positive or negative according as it is joined to the -positive or negative pole of the current generator. By convention, -electricity is supposed to "flow" from the positive pole of the battery -to the positive electrode or _anode_, and then through the solution to -the negative electrode or _cathode_, and so back to the negative pole of -the generator, thus completing the circuit external to the battery. - -When acids, alkalis, and salts are dissolved in water, there is strong -evidence to show that they break up to a greater or less extent into at -least two parts called _ions_. These are atoms, or groups of atoms, -which have either acquired or lost one or more _electrons_.[5] They move -about quite independently of one another and in any direction until the -electrodes are placed in the liquid. Then they are constrained to move -in two opposing streams--those which have acquired electrons all move -towards the negative electrode, and those which have lost electrons -towards the other. At the electrodes themselves, the former give up and -the latter take up electrons, and become atoms again. Let us now -consider a concrete example. Common salt is composed of atoms of sodium -and atoms of chlorine paired. When a small quantity of this substance is -dissolved in a large quantity of water, the pairing no longer obtains. -The chlorine atoms move away independently accompanied by an extra -satellite or electron, and the sodium atoms move away also but with -their electron strength one below par. When the current is introduced -into the liquid, the sodium ions travel towards the cathode and chlorine -ions towards the anode, and when they reach the goal, sodium ions gain -one electron and chlorine ions lose one, and both become atoms again. -Chlorine atoms combine in pairs forming molecules and escape from the -solution in the greenish yellow cloud that we call chlorine gas. The -sodium atoms react immediately with water, forming caustic soda with the -liberation of hydrogen. - -To return now to practical considerations. The electrolysis of salt -solution appears to be an ideally simple method of obtaining caustic -soda and chlorine from sodium chloride. As a manufacturing process, it -would seem to be perfect, for the salt is broken up directly into its -elements and a secondary reaction gives caustic soda automatically. -There is no "waste" as in the Leblanc process, and it does not require -the use of any expensive intermediary substance afterwards to be -recovered, as in the Solvay process. But, as very often happens when -working on a large scale, difficulties arise, and these up to the -present have only been partially overcome. - -Some of the chlorine remains dissolved in the liquid and reacts with the -caustic soda, forming other substances which, though valuable, are not -easy to separate from the caustic soda. It is possible to get over this -difficulty to some extent by placing a porous partition between the -anode and the cathode, and in that way dividing the cell into cathodic -and anodic compartments. As long as the partition is porous to liquids, -it will allow the current to pass, but at the same time it will greatly -retard the mixing of the contents of the two compartments. Porous -partitions or cells which are in common use for batteries are made of -"biscuit" or unglazed porcelain. - -It must be remembered, however, that porous partitions only retard the -mixing of liquids; they do not prevent it. Moreover, a further -difficulty arises from the fact that chlorine is a most active -substance, and therefore it is difficult to find a material which will -resist its corrosive action for any length of time, and the same -difficulty arises in the case of the anode where the chlorine is given -off. - -Castner Process for Caustic Soda. The following is the most successful -electrical process for the manufacture of caustic soda yet devised. It -was introduced in 1892, and is known as the Castner process. It should -be noted that the use of the porous partition has been avoided in a very -ingenious way. - - [Illustration: Fig. 16. THE CASTNER PROCESS] - -The cell (see Fig. 16) is a closed, rectangular-shaped tank divided into -three compartments by two non-porous partitions fixed at one end to the -top of the tank, while the other end is free and fits loosely into a -channel running across the tank. The floor of the tank is covered with a -layer of mercury of sufficient depth to seal the separate compartments. -The two end compartments contain the brine in which are the carbon -anodes; the middle compartment contains water or very dilute caustic -soda in which the cast-iron cathode is immersed. - -The current enters the end compartments by the carbon anodes and passes -through the salt solution to the mercury layer which in these -compartments are the cathodes. The current then passes through the -mercury to the middle compartment, and then through the solution to the -cathode, thence back to the dynamo. It is important to note that in the -middle compartment the mercury becomes the anode. - -Chlorine is liberated at the carbon electrodes, and when no more can -dissolve in the liquid it escapes and is conveyed away by the pipe P. -Sodium atoms are formed at the surface of the mercury cathodes in the -outside compartments and dissolve instantly in the mercury, forming -sodium amalgam. - -While the current is passing, a slight rocking motion is given to the -tank by the cam E. This is sufficient to cause the mercury containing -the dissolved sodium to flow alternately into the middle compartment, -and there the sodium amalgam comes into contact with water; the sodium -is dissolved out of the mercury and caustic soda is formed. Water in a -regulated stream is constantly admitted to the middle compartment, and a -solution of caustic soda of about 20 per cent. strength overflows. - -The production of caustic soda by an electrical method still remains to -be fully developed. A process which gives only a 20 per cent. solution -cannot be looked upon as final. In the meantime, other methods have been -tried, in some of which fused salt is used in place of brine in order to -give caustic soda in a more concentrated form. For a description of -these methods, the reader must consult some of the larger works -mentioned in the preface. Here we can only say that very great -difficulties have been encountered, particularly in the construction of -a satisfactory porous diaphragm or, alternately, in devising methods in -which this can be dispensed with. - -Another interesting application of electrolysis is furnished by the use -of copper sulphate in industry. When this salt is dissolved in water, it -breaks up into copper ions (positive) and an equal number of negative -ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO"4). Under -the influence of the current copper ions travel to the cathode, and -there by the gain of two electrons become copper atoms. Now, since -copper is not soluble in copper sulphate solution, and is not volatile -except at very high temperatures, it is deposited on the cathode in a -perfectly even and continuous film when the strength of the current is -suitably adjusted. This film continues to grow in thickness as long as -the conditions for its deposition are maintained. If the current -employed is not suitable, the metallic film is not coherent, and the -copper may appear as a red powder at the bottom of the cell. Any other -metal or impurity which might be present in the unrefined copper falls -to the bottom of the tank. - -Other metals are deposited electrolytically in exactly the same way. The -metal to be deposited is joined to the positive pole and the article to -be plated to the negative pole of the battery. Both are suspended in a -solution of salt, generally the sulphate, of the metal which is to be -deposited. Thus, for nickel plating, a piece of sheet nickel would be -used in conjunction with a solution of sulphate of nickel or, better, a -solution of nickel ammonium sulphate, made by crystallizing ammonium and -nickel sulphates together. The current required is small; indeed, if it -is too strong, the deposit adheres loosely to the article, and the -result is, therefore, not satisfactory. - -Electrotype blocks are also made by a similar process. An impression of -the article to be reproduced is made in wax, or some suitable plastic -material, and polished with very fine graphite or black lead, in order -to give a conducting surface. It is then suspended in a solution of -copper sulphate and joined to the negative pole of the battery; a plate -of copper connected with the positive pole is suspended in the same -solution. When a weak current is passed, copper is deposited on the -black-leaded surface and grows gradually in thickness, until at length -it can be stripped off, giving a positive replica of the object. - - - - - INDEX - - - A - Acetic acid (glacial), 73 - Acids, early notions of, 1 - ----, fatty, 78 - ----, mineral, 68 - ----, vegetable, 68 - Agate, 61 - Air-saltpetre, 42 - Alkali Acts, 44 - ----, caustic, 96 - ----, metals, 95 - ----, mild, 80 - ---- waste, 87 - Alkalis, properties, 3 - Aluminium acetate, 73 - Alums, the, 26 - Amethyst, 61 - Ammonal, 36 - Ammonia, 97 - ----, synthetic, 99 - Ammonite, 99 - Ammonium carbonate, 99 - ---- chloride, 98 - ---- nitrate, 99 - ---- sulphate, 99 - Anhydride, an, 21 - Anode, 103 - Argol, 76 - Asbestos, 63 - ----, platinized, 19 - Ash, black, 84 - ----, pearl, 93 - ----, soda, 10, 92 - Atolls, 51 - Atomized water, 18 - - - B - Bacon, Roger, 32 - Basic slag, 58 - Basil Valentine, 12 - Beryl, 63 - Black liquor, 74 - Blasting gelatine, 35 - Bleaching powder, 46 - Blue-john, 47 - Boiler scale, 54 - Bonbonnes, 31 - Bone, 56 - ---- ash, 57 - ---- black, 56 - ---- meal, 56 - Borax, 59 - Bordeaux mixture, 7 - Boric acid, 58 - Boyle, Robert, 2 - Burgundy mixture, 6 - - - C - Calcium acetate, 5 - ---- bicarbonate, 54 - ---- carbonate, 50 - ---- fluoride, 47 - ---- nitrate, 29 - ---- phosphate, 56 - ---- sulphate, 27 - Calc spar, 50 - Caliche, 29 - Calico printing, 26 - Carbon, 49 - Carbonic acid, 49 - ---- ---- gas, 49 - Castner process, 105 - Catalytic action, 20 - Cathode, 103 - Cat's-eye, 61 - Cavendish, H., 40 - Cellulose, 46 - Chalcedony, 61 - Chalk, 50 - Chert, 66 - Chili-saltpetre, 29, 39 - China clay, 62 - Citric acid, 77 - Chlorides, 47 - Chlorine, 46 - Chrome yellow, 28 - ---- red, 28 - Compound, 7 - Compounds, binary, 8 - Contact action, 20 - ---- process, 18 - Copper refining, 107 - ---- sulphate, 5, 27 - Coral reefs, 51 - Cordite, 34 - Cream of tartar, 76 - Crops, rotation of, 37 - Crystallization, water of, 9 - Crystals, 9 - - - D - Davy, Sir Humphry, 95 - Derbyshire spar, 47 - Devitrification, 65 - Dynamite, 35 - - - E - Efflorescence, 82 - Electrode, 103 - Electrolysis, 102 - Electrons, 103 - Electrotype blocks, 107 - Element, definition of, 7 - Elements, list of, 8 - Explosives, 32 - - - F - Felspars, 62 - Ferrous acetate, 74 - ---- sulphate, 25 - Flint, 61 - Fluorspar, 48 - Formic acid, 78 - Fur in kettles, 54 - - - G - Garnet, 63 - Gas, laughing, 99 - ---- lime, 12 - ---- liquor, 98 - Gay Lussac tower, 16 - Glass, 64 - ----, annealing of, 65 - ----, Bohemian, 63 - ----, etching on, 47 - ----, flint, 63 - ----, lead, 63 - ----, soda, 63 - ----, water, 66 - Glauber's salt, 10 - Glover tower, 17 - Glue, 56 - Graphite, 108 - Greek fire, 32 - Guncotton, 34 - Gunpowder, 32 - Gypsum, 27 - - - H - Haber process, 100 - Halogen, 43 - Hardness, permanent, 53 - ----, temporary, 53 - Hartshorn, salt of, 99 - ----, spirits of, 97 - Hornblende, 63 - Hydriodic acid, 48 - Hydrobromic acid, 48 - Hydrochloric acid, 43 - Hydrofluoric acid, 47 - - - I - Iceland spar, 50 - Ions, 103 - Iron pyrites, 11 - - - J - Jade, 63 - Jasper, 61 - - - K - Key industries, 10 - - - L - Lake, 26 - Lead acetate, 75 - ---- chambers, 17 - ---- chamber process, 14 - ----, sugar of, 75 - ---- sulphate, 27 - ----, white, 75 - Leblanc soda process, 82 - Leguminosae, 37 - Lemon, salts of, 77 - Lime burning, 51 - ----, caustic, 97 - ---- kiln, 51 - Limestone, 50 - Litmus, 2 - Lupin root, 37 - - - M - Marble, 50 - Marking ink, 28 - Meerschaum, 63 - Mica, 63 - Mordants, 26 - Mycoderma aceti, 68 - - - N - Neutralization, example of, 4 - ----, explanation of, 3 - Niagara, 101 - Nitre, 29 - ---- pots, 14 - Nitric acid, 30 - ---- ----, from air, 40 - ---- ----, importance of, 28 - ---- ---- manufacture of, 30 - ---- ----, properties, 31 - ---- ----, red fuming, 31 - ---- oxide, 16 - Nitrogen cycle, 37 - ----, fixation of, 100 - ---- peroxide, 16 - Nitroglycerine, 34 - - - O - Olein, 78 - Onyx, 61 - Opal, 61 - Orthoclase, 62 - Oxalic acid, 77 - - - P - Palmitin, 78 - Pearls, 51 - Peregrine Phillips, 21 - Philosopher's stone, 2 - Phosphoric acid, 57 - Plaster of Paris, 27 - Potash, caustic, 97 - ----, mild, 93 - Potassium, 95 - ---- bicarbonate, 94 - ---- nitrate, 29 - Propellants, 33 - Prussian blue, 25 - Pyrites burners, 14 - Pyroligneous acid, 73 - - - Q - Quartz, 61 - ---- fibres, 62 - ----, smoky, 61 - Quicklime, 5, 51 - - - R - Red liquor, 73 - Rock crystal, 61 - Rupert's drops, 65 - - - S - Sal ammoniac, 99 - ---- prunella, 29 - Salt cake, 84 - ----, common, 47 - ----, formation of a, 4 - Saltpetre, 29 - Salts, from carbonates, 5 - ----, from oxides, 5 - ----, from metals, 4 - ----, insoluble, 6 - Sandstone, artificial, 66 - Saponification, 79 - Schweinfurt green, 27 - Shells, egg, 51 - ----, oyster, 51 - Silica, 61 - ---- ware, 62 - Silicic acid, 62 - Silver bromide, 48 - ---- chloride, 48 - ---- iodide, 48 - ---- nitrate, 28 - ---- sand, 61 - Soap, hard, 79 - ----, soft, 79 - Soda, baking, 88 - ----, bicarbonate of, 6, 88 - ----, bread, 88 - ----, caustic, 96 - ----, mild, 80 - ----, natural, 82 - ----, washing, 3, 5, 81 - ---- water, 49 - Sodium, 95 - ---- nitrate, 29 - ---- sulphate, 27 - Soil bacteria, 38 - Solvay process, 90 - Sorrel, salts of, 77 - Spent oxide, 11 - Stalactite, 53 - Stalagmite, 53 - Stearin, 78 - ---- candles, 79 - Stone ammonia, 99 - Suffioni, 60 - Sulphur, 11 - ---- dioxide, 11 - ---- trioxide, prep. of, 19 - Sulphuric acid, properties, 20, 24 - ---- anhydride, 21 - Sulphurous acid, 11 - Superphosphate, 57 - - - T - Tallow, 79 - Tartaric acid, 76 - Tinkal, 61 - Trinitrotoluene, 35 - - - V - Verdigris, 74 - Vert de Montpellier, 74 - Vinegar, 68 - ----, malt, 70 - ----, wine, 70 - Vitriol, blue, 5 - ----, nitrated, 16 - ----, oil of, 12 - - - W - Ward, Dr., 12 - Water, hard, 53 - ----, soft, 53 - ----, softening of, 54 - Wood ashes, source of potash, 3 - ---- ----, used as soap, 2 - - - Z - Zinc chloride, 5 - - - THE END - - - - - Footnotes - - -[1]An anhydride is a substance which unites with water to form an acid. - -[2]See Frontispiece. - -[3]Now £13 a ton. - -[4]Basic lead carbonate. - -[5]An electron is probably an "atom" of negative electricity detached - from matter. - - - _Printed by Sir Isaac Pitman & Sons, Ltd. Bath, England_ - (v--1468c) - - - - - Transcriber's Notes - - ---Silently corrected several palpable typographical errors. - ---Retained publication information from the original source. - ---In the text versions, included italicized text in _underscores_. - - - - - - - -End of the Project Gutenberg EBook of Acids, Alkalis and Salts, by -George Henry Joseph Adlam - -*** END OF THIS PROJECT GUTENBERG EBOOK ACIDS, ALKALIS AND SALTS *** - -***** This file should be named 50552-8.txt or 50552-8.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/0/5/5/50552/ - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net (This file was -produced from images generously made available by The -Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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