Popular Science Monthly/Volume 25/June 1884/Coal and the Coal-Tar Colors

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Popular Science Monthly Volume 25 June 1884  (1884) 
Coal and the Coal-Tar Colors
By Denys Cochin



WITHIN thirty years, the agriculture of some countries has been subjected to an unprecedented competition. Vegetable productions identical with those they were accustomed to furnish have been extracted from stone-coal. Coal was at first employed only as a combustible; then it gave us gas and illuminating oils. Now it furnishes us perfumes and colors; the flavors of bitter-almonds and of vanilla, and the orange-red of madder, which is no longer cultivated around Avignon. We derive from coal what we used to look for in living plants, and the art of the chemist has fabricated vegetable substances. It would not, however, be correct to say that vegetable substances have been constituted from mineral elements, for coal is not a mineral, but a decomposed vegetable product. It is not pure carbon, but a mixture of hydrocarbons, of combinations which chemistry calls organic, because they proceed from living organisms and preserve a distinctive character peculiar to substances that have been endowed with life. It is not, then, the mineral world that yields us the perfumes and colors that were furnished by plants and flowers, but an intermediate world in which the remains of the vegetation of past ages are preserved.

If we heat bituminous coal in a close vessel communicating with cooled receivers, we shall have carbon left in our retort, mixed with a little sulphuret of iron. This is coke. The products of the distillation that pass over will be of two kinds; a thick liquid, coal-tar, and carbureted hydrogen gases. The gases are used for lighting. Thirty years ago the coal-tar was not used for anything. We shall proceed to inquire what profit is now derived from it. What is it precisely that takes place in the retort? Shall we believe that the light and 202 THE POPULAR SCIENCE MONTHLY,

spongy coke was a kind of skeleton of coal intimately united with more complex substances, and that coal is a mixture of pure carbon and combined carbon ? No ; coal, as a whole, is a mass of substances composed of combinations of carbon with other bodies. These com- binations are modified by heat. The tarry liquids and the gases do not exist in the coal, but are formed as the temperature rises in the retort. Coke is left, because in the changes that are made carbon is in excess. The coal-tar is not separated from the coke, but is made in the retort, and the bodies we find in it are results of combinations that are brought about between the substances which existed in the coal.

M. Berthelot heats to a dull red heat the gas acetylene, the mole- cule of which is composed of four atoms of carbon and two atoms of hydrogen. At the end of the operation the acetylene is condensed and is changed into a liquid, benzine, which is composed of twelve atoms of carbon and six of hydrogen. Three molecules of acetylene have been in some way welded together to furnish a molecule of ben- zine. We have seen acetylene condensed and combined as it were with itself. It also combines with hydrogen and forms olefiant gas, or ethylene. The latter unites with the benzine and gives, by syn- thesis, a liquid hydrocarbon, styrolene, identical with the styrolene which is produced by the sty rax or Oriental liquidamber. Finally, from the union of the styrolene and the olefiant gas results naphtha- line, a solid hydrocarbon, which crystallizes in thin lamellae and abounds in coal-tar. Anthracene is one of the most valuable of the hydrocarbon extracts of coal-tar. It evidently did not exist as anthra- cene in the coal, but has been formed during the distillation, a solid, crystalline body, by the combination and condensation of gases.

So, when coal is heated to a very high temperature, the substances that are disengaged in a gaseous form do not always remain in that state. Heat is not always a cause of the dissolution of bodies and of the dispersion of their elements. When exposed to a temperature ex- ceeding 1,000° C. (1,800° Fahr.) these gases condense ; their molecules draw together ; and they form, after a few changes, combinations richer in carbon, and consequently less volatile. We had gases, but, when our apparatus has had time to cool, we shall find liquids, even crystals. In other cases, dissociation is effected by heat. Carbonic acid, one of the most common and stable compounds in the world, the final resultant of all combustion, loses its oxygen under excessive heat, and becomes an oxidizing agent. In this way good authorities explain the production of phenic, acetic, and cresylic acids, as hydrocarbons oxidized by the oxygen of carbonic acid. The hydrocarbons may also be dissociated. A liquid hydrocarbon analogous to benzine, toluene, takes hydrogen and leaves a deposit of anthracene. Formene, or marsh-gas, a hydrocarbon which produces chloroform when the hydro- gen in it is replaced with chlorine, loses hydrogen and yields anthra- cene.


Sometimes contrary forces are developed simultaneously, and bod- ies are at the same time subjected to an influence which brings them together and to another one which separates them ; the result will depend upon slight diifferences in the temperature or in the propor- tions of the different bodies present. Benzine and carbonic acid unite to form benzoic acid ; benzoic acid decomposes into carbonic acid and benzine. Styrolene is produced by the union of benzine and olefiant gas, and in decomposing yields benzine and acetylene. Benzine makes its appearance again if anthracene and naphthaline are heated in the presence of hydrogen. Sometimes, between these contrary forces, an equilibrium is established. Thus, acetylene will combine with hydro- gen and form olefiant gas ; but olefiant gas will decompose at the same temperature, giving out its two elements ; while, if the three gases are present and all pure, action will be suspended, for the opposing tendencies will be counterbalanced.

These are only a few of the examples of the reactions that take place when organic substances are raised to a high temperature. The four simple substances entering into the constitution of organic bodies form among one another more compounds than are furnished by all the minerals. If heating takes place in the open air, combustion en- sues, and all these innumerable substances are oxidized and dissipated in the atmosphere as carbonic acid and aqueous vapor. But, if we work in a medium free from oxygen and all other foreign elements, they react upon one another, and a multitude of bodies are formed or decomposed by the interchange of elements, and the mixture we get when the heat is removed is a mixture of new elements. So, solid and dry coal gives the coal-tar liquids and illuminating gas, which did not exist in it, but were formed under the influence of heat.

Of what organic substances coal is really composed we know only imperfectly. Chemists have not succeeded in making real analyses of it. We can tell how much of impurities, such as sulphuret of iron, it contains, and how much coal-tar and gas can be got from it ; we may classify a specimen as a rich, a poor, or a bituminous coal, or as one giving a long or a short flame, but we do not separate and deter- mine the chemical elements.

The analyst has not very many resources at his disposal for sepa- rating an intimate mixture of several bodies. The first means is that of distillation. Different bodies sublime at different temperatures, according to their various degrees of volatility ; each of them, under the same atmospheric pressure, passes from the solid to the liquid state at one temperature, and from the liquid to the gaseous at an- other. These temperatures are called, respectively, the point of fusion and the boiling-point. Fractional distillations are performed in ac- cordance with this principle. When the heat is raised to a certain degree, one class of bodies, at a higher temperature another class of bodies, which had not reached their boiling-point at the former temper-


ature, will be collected in the cold receiver. The operation becomes complicated and the results perplexing when the mixture consists of substances capable of being modified by the degrees of heat applied. In such cases the analysis must be carried on at a lower temperature, and the operator must depend upon solvents, the effects of which are different on different bodies. This method has been tried on coal by M. Commines de Marcilly, who employed boiling liquids or their vapors in open and in closed vessels, and in Papin's digester, by the aid of which he obtained a stronger pressure than that of the atmosphere. Acids and alkalies had no action, but neutral liquids, such as ether, benzine, sulphuret of carbon, and chloroform, were evidently colored by the coal. The experiments deserve to be carried further.

Coal-tar, the liquid product which is formed when coal is roasted in a close vessel, appears as a thick, black paste, giving no hint of the richness of the substances which may in their turn be formed and separated from it. The first product, water saturated with ammonia, passes over when the liquid is heated to between 175° and 192° Fahr. for twenty or thirty hours. Then a fractional distillation is performed, under which the light oils are separated at below 266° Fahr. ; the me- dium oils at between 266° and 392° ; and the heavy oils at between 392° and 678° ; while a thick residue is left in the retort. Our study is with the oils.

The first two classes of oils are again distilled in a large alembic heated by steam under high pressure ; first is collected for the medium oils all that passes between 266° and 392°. That which passes at be- low 266° is mixed with light oils, while the products passing at above 392° are mingled with heavy oils. The light oils are next purified in a similar manner. The latter products are known in commerce as naphtha-oils, and are chiefly carburets of hydrogen. The eighteen or twenty of them which have been distinguished form a series, in which the proportion of carbon to hydrogen increases regularly. Those least rich in carbon are gaseous ; then come the liquid hydrocarbons, and last the solid compounds. We select the liquid distillates for further operations. The first step is to rid the product of the gases that may still be dissolved in it, and the alkaline or acid impurities it may con- tain — foreign matters which give to the naphtha a repulsive odor. They are separated by washing successively with water, which re- moves some of them, sulphuric acid, which acts on the alkalies, and caustic soda for the removal of acids and what excess of sulphuric acid may remain. The naphtha is then subjected to a fourth distilla- tion, and benzine is obtained at a temperature of between 184° and 240°.

Before proceeding with the history of this valuable substance we will mention that the medium oils are treated with sulphuric acid and soda in the same way as the light oils, except that, as they are richer in alkalies and acids, they have to be treated with stronger proportions


of the cleansing agents. They are then put into the market as illu- minating oils. They may also be used for solutions of India-rubber, but sulphuret of carbon is preferred for that purpose.

Faraday discovered benzine in 1825 among the products arising in the manufacture of oil-gas, and called it bicarbureted hydrogen. Mitscherlich, in 1825, in treating benzoic acid with soda, obtained a volatile liquid which he called benzine. Hofmann, in 1825, demon- strated that these two substances were the same. Berthelot explained the formation of the substance, and made a synthesis of it by heating acetylene, its molecule being composed of three molecules of that gas united, or of twelve atoms of carbon and six of hydrogen. Benzine is a type of a class of organic bodies that furnish, by substitution, innumerable series of derivatives. They are like buildings from which we can take the stones one at a time and replace them with others. They are the organic radicals, in which a number of atoms of carbon and hydrogen are associated in such a way that the energy of one atom of hydrogen is left free. In benzine, for instance, we may sub- stitute for each atom of hydrogen an atom of chlorine and get benzine monochloride, benzine dichloride, etc., or an atom of bromine or iodine and get benzine bromide and benzine iodide ; or another radical, such as methyl or ethyl, and get methylbenzine or ethylbenzine, dimethyl- benzine, trimethylbenzine, and so on. These theories permit us to account for the long series of bodies which organic chemistry has re- vealed, many of which are now employed in industry.

Benzine, as everybody knows, is a light liquid, perfectly colorless, and having a nauseous odor. It nevertheless furnishes perfumes and dyes. Charles Mansfield, who was the first person to utilize benzine, and make it on a large scale, announced in 1847 that he had found among the derivatives of stone-coal an oil that might take the place of the oil of bitter-almonds. It was nitrobenzine. Mitscherlich had pre- viously produced, by the lively reaction of nitric acid on benzine, a colorless liquid, in which a compound molecule of nitrogen and oxygen was substituted for one of the six atoms of hydrogen in benzine, but his experiment never got beyond the laboratories. It was attended by too great dangers. ^Nevertheless, Mansfield ventured to repeat it in his shop, and succeeded in basing an industrial operation upon it. Nitrobenzine can not be pure unless the benzine was pure, and that is rarely the case with the commercial article. In the mixture of hydro- carbons, of which naphtha is constituted, are some very nearly alike in composition and in respect to their boiling-point, and it is difficult, even with the best distilling apparatus, to arrest the passage of some of them. Toluene, for example, nearly always comes over with ben- zine. Like it, it is attacked by nitric acid and then yields a nitro- toluene. There has also been found, associated with nitrobenzine, a peculiar yellowish-colored acid, endowed with the smell and taste of the pineapple ; and its ethers taste like the strawberry or the rasp-


berry. It has given the flavor to many a sherbet and many a confec- tion.

Nitrobenzine is known in trade under the purely fanciful name of essence of mirbane, and is used by perfumers as a substitute for the oil of bitter- almonds — a substance which is also made artificially. It plays an important part in modem industry, because it is employed in the manufacture of aniline.

As the experiments in synthesis are continued, and more and more complicated bodies are evolved from the primitive hydrocarbon, the wealth of the field of researches open to the investigator becomes more and more surprising. How many combinations have already been effected, and how many thousand remain to be discovered ! Benzine is only one of many hydrocarbons derived from coal-tar, and nitroben- zine is only one of the nitrogenized derivatives from it. There are also iodine, bromine, and chlorine derivatives, which may be obtained, not only by successive substitutions of those substances for one or more atoms of hydrogen, but also by additions of them, without dis- placing hydrogen. Sulpho-derivatives are also known, as well as ni- trogenized derivatives of benzine chloride, iodide, and bromide. In- stead of chlorine, iodine, and bromine, we may substitute organic radicals for hydrogen and get other new series. And these series of derivatives furnished by benzine are paralleled by other like series derived from toluene, xylene, and a hundred other hydrocarbons. Mathematicians exhibit a formidable total of the different possible ar- rangements according to which the units may be grouped by twos and threes, etc. ; the seven notes of the musical scale are arranged in in- finite variations ; and chemistry disposes the seven or eight bodies occurring in organic matters in a similar endless diversity of combina- tions. If we are permitted to extend the comparison, we may say that as the musical arrangements are based upon a certain fundamental chord, so types of chemical arrangements center around a particular model, like benzine, to which it is easy to bring the whole series into relation.

Aniline exists already formed in coal-tar, but in very small quan- tity. Industry does not look after it, for the processes of extraction would be too costly. It is more convenient to make nitrobenzine and then reduce it, or deprive it of its oxygen by bringing it in contact with substances that will take that element from it. This may be effected by several processes. Sulphureted hydrogen, iron in fine par- ticles, and acetic acid, are often employed as reducing agents. All the substances we have thus far derived from coal-tar are colorless. The moment has come for colors to appear. We have obtained aniline by deoxidizing nitrobenzine. If we are expecting in turn to recover nitrobenzine by oxidizing aniline, we shall find ourselves mistaken. We can, indeed, fix oxygen upon the hydrogen, but the hydrogen- atoms will separate during the process from the molecule of aniline.


Not a fixation of oxygen, but a departure of hydrogen, takes place. Then a phenomenon of condensation is exhibited ; a number of the molecules unite to form a molecule of rosaniline. This wonderful col- orant may be constituted by the action of almost any of the oxidizing agents known in chemistry upon aniline. Curiously, rosaniline would not be formed if the aniline were absolutely pure. Theoretically, its molecule is formed by the union of a molecule of aniline and two molecules of toluidine, with a loss between the two of six atoms of hy- drogen. It can not be obtained by oxidizing either of these bases separately. Rosaniline is solid at ordinary temperatures, and crys- tallizes readily in lozenges or in fine needles, which are white when protected from the air, but become rose and then red when brought in contact with it. The nature of the change it undergoes is unknown. It is not apparent in the composition. Rosaniline is soluble in water, and more soluble in alcohol, and has basic qualities so strong as to dis- place ammonia from its salts ; and it is most frequently employed as a salt. It furnishes not red only, but all colors, according as it is treated in the combinations into which it is made to enter. Violet was first discovered by Mr. Perkins, in 1856, while trying to make ar- tificial quinine by the action of bichromate of potash on sulphate of aniline. He gave up the search for quinine, and turned his attention to manufacturing the color. Three years afterward MM. Renard and Verguin produced fuchsine, a purple salt of rosaniline, by treating commercial aniline with a dehydrogenizing agent, bichloride of tin. It is a mixture of hydrochl orate of rosaniline and salts of tin, and is used by dyers and wine-merchants. Aniline is now oxidized by the ac- tion of arsenic into crude red {rouge brut), a violet mixture, composed principally of arsenite and arseniate of rosaniline, which is converted into fuchsine by bringing about a substitution of hydrochloric for arsenious or arsenic acid. This is done by boiling crude red with hy- drochloric acid, or, more usually, with sea-salt. A double decompo- sition takes place, and, when the liquor is cooled, crystals of fuchsine are found in the bottom of the vessel, while the arsenites and arseni- ates of soda are retained in the mother- water. INot all the coloring- matter, however, is deposited in the crystals, and a good operator loses nothing. Treated with carbonate of soda, the mother- water gives a precipitate, from which is extracted a color known as aniline garnet or yellow fuchsine. Nor is this all. The crude red has left a violet deposit in the bottom of the boats in which it was cooled ; this is washed in boiling water ; the water is colored red, and a blue dye- stuff is collected from it. More is left still. The crude red has passed through filters, and they have retained some insoluble substances. These are carefully gathered up ; they form a paste which is boiled with diluted hydrochloric acid and filtered over again to extract what fuchsine is left. The insoluble residue furnishes aniline ma- roon, a beautiful color readily applicable to wool. Thus a single


operation has given us the violet red of fuchsine, garnet, blue, and maroon.

Whence come all of these colors ? And how does chemistry ex- plain the provision of so various hues by the same body ? The differ- ences do not arise solely from the fact that the same base, rosaniline, is found associated with different acids. We must not forget that we had at first, notwithstanding the separations effected by fractional dis- tillations, a mixture of substances. These substances react upon one another ; and the theory of their reactions, of which we have already given some idea, appears so ingenious and interesting that we must say a few words more about it.

Benzine and toluene, mixed, furnished, after some reactions, a mixt- ure of aniline and toluidine. Two molecules of toluidine and one molecule of aniline united, with a loss of hydrogen, to form a mole- cule of rosaniline. Now, two molecules of aniline and one molecule of toluidine, also losing hydrogen, might also unite in a similar man- ner ; or three molecules of aniline, or three molecules of toluidine, might be introduced in the process, with analogous results. Here we have four distinct arrangements, four possible cases, conceived in theory and realized in practice. In the first case we had rosaniline ; in the second, we have mauvaniline ; in the third, violaniline ; and, in the fourth, chrysotoluidine. We have described the first of these substances. The second forms light-brown crystals, that become darker on heating, while the liquids in which they are dissolved take a violet tinge. Violaniline is hardly soluble, and difficult to get crys- tallized ; it is a' very dark — nearly black — brown powder. Its salts, when a few drops of concentrated sulphuric acid are added to the solution, give a dark blue. Chrysotoluidine is yellow. All these bodies are formed during the preparation of fuchsine, and are sepa- rated by filtration or through their differences in solubility, or inca- pacity for crystallization. The separation of the substances which do not crystallize is difficult and incomplete. The red continues united with the yellow in greater or less proportion, and gives maroon or garnet.

Through all these processes, in which we have observed the hydro- carbons decomposing one another, and forming new compounds, we have found that the chemistry of coal does not always have to borrow its powerful reagents, its acids and alkalies, from mineral chemistry ; but that the compounds of carbon themselves, closely allied in consti- tution and properties, are very frequently capable of reacting upon and transforming one another, without the intervention of foreign agents. Instead of acids uniting with bases to give rise to a third kind of bodies, salts, we have carburets, bases, uniting by twos or by threes, with or without the loss of one of their elements, and forming double or triple molecules of compounds, which may still be of the same chemical type. The first experiments in the practical applica-


tion of these reactions were made by MM. Charles Girard and De Laire. Chemists, as we have said, understand by organic radicals cer- tain groups of atoms of carbon and hydrogen, which are capable of combining with an atom of hydrogen in the same manner as an atom of bromine, or iodine, or chlorine, or which may be substituted for an atom of one of these substances in one of its combinations. In a complex body like rosaniline, one or more atoms of hydrogen may be removed and replaced by as many atoms of the organic radical. MM. Girard and De Laire caused aniline to react upon rosaniline. Aniline is an organic base, an ammoniacal compound. In common ammonia, one atom of nitrogen is combined with three atoms of hydrogen. In aniline, one of the atoms of hydrogen is replaced by the radical phenyle. The converse is also possible, and, if phenyle is in its turn replaced by hydrogen, the ammonia should reappear. This reaction was provoked by heating fuchsine and aniline together. Rosaniline gave up an atom of hydrogen and took the radical phenyle. Aniline lost phenyle, which was replaced by hydrogen ; the ammonia was dis- engaged, and phenyl-rosaniline was produced. It is a bright sky-blue. We can vary its color. The exchange we have just described may be effected successively for three atoms of hydrogen against three molecules of phenyle, according to the amount of aniline employed ; and we shall have monophenyle, diphenyle, or triphenyle rosaniline. The first is violet-blue, the second clear-blue, and the third a blue we might call blue-light {hleu lumiere), because its hue loses none of its freshness — and, in fact, gains luster — even in an artificial light.

MM. Girard and Laire's discovery was of great theoretical and practical interest, and important consequences followed it. The method was general, and permitted the substitution, in most of the organic bases, of radicals for two or three atoms of hydrogen. The same chemists succeeded in doing with the hydrochlorate of aniline as they had done with the hydrochlorate of rosaniline, and obtained diphe- nylated and triphenylated aniline, from which they extracted blue coloring matters ; then they brought the salts of these complex bases under the review of their experiments. An iodine salt of trimethylated rosaniline gave them a magnificent green, of such fixity and luster that it might be called, like the blue which they had previously pre- pared, green-light {vert lumUre).

The light oils of coal-tar are almost wholly composed of carburets of hydrogen ; in the heavy oils bases and acids are also found with some very condensed carburets. They contain, for example, the ready- formed aniline, which it has not been found profitable to extract from them, and phenic acid, which, besides its valuable antiseptic proper- ties, has been serviceable to the fabricants of coloring matters. In 1834 M. Runge, in preparing phenic acid, found in the residue a yel- low substance, which is called coralline, or rosolic acid. In 1859 M. Jules Persoz, heating this substance with ammonia, obtained a beau-

YOL. XX7. — 14


tiful red body, which he called peonioe. Two years afterward, the manufacturers to whom he sold his patents put in the market a sky- blue substance, azuline, which also was a derivative of rosolic acid. The precise nature of rosolic acid has not been determined ; but M. Fresenius has extracted an orange-yellow matter from it, which he calls aurine, and has devised a process for procuring it by heating phenic and sulphuric acids together, and adding oxalic acid six or seven hours afterward. It is not much used now, but is of interest as furnishing in itself blue, red, and yellow. The most important bodies derived from the heavy oils are naphthaline and anthracene, both car- burets of hydrogen. Naphthaline, which is solid at ordinary tem- peratures, and crystallizes with great facility in thin lamellsB, is ob- tained simply by leaving the oils in the cold for five or six days, when it becomes solidified. The liquid is decanted off, and the crystals are pressed, to remove the included oil, into thick cakes. Naphthaline is a member of the same series as benzine, and is subject to a similar series of reactions. Reducing its nitrate, Zinin obtained an organic base analogous Xio aniline, naphthylamine, which is transformed by the loss of hydrogen into rosanaphthylamine. From this is obtained the hydrochlorate of rosanaphthylamine, a body analogous with fuch- sine, of a beautiful rose-color, and easily crystallizable, but of a clearer rose with less of violet than fuchsine. It is dull when applied to wool, but gives very brilliant hues with silk. Dissolved in alcohol, it produces a strange and wonderful effect. The liquid turns bright red, and, under proper presentation to the light, may be seen to be traversed with phosphorescent clouds. If left to stand till the alcohol has slowly evaporated, the bottom of the vessel will be covered with beautiful green, iridescent needles. Naphthaline also furnishes some very com- plex compounds, whence have been derived very yellow dyes, among which Manchester yellow and Martins yellow are the best known. Experiments in substituting molecules of organic radicals for atoms of hydrogen, as has been done with rosaniline, have been made with some success, but the blues thus obtained have not the remarkable fixity and luster of the similar rosaniline products.

No discovery of coal-tar products is more extraordinary or more fruitful in its bearings than that of the extraction of alizarine, or the artificial preparation of the coloring principle of madder, the effect of which has been to work a real economical revolution, and to destroy the most profitable agricultural industry of large districts of country. The madder-root has furnished the most generally used of all dye- stuffs, and the one which constituted the basis of nearly all our colors. The substance to which it owes its peculiar virtues still performs the same functions, but, instead of being derived from the cultivated root, it is now procured by chemical synthesis from stone-coal.

Alizarine is prepared from anthracene, the second of the more im- portant bodies which we have already spoken of as contained in the


heavy oils of coal-tar. It forms a part of the deposit of solids which forms when the heavy oils are left standing in the cold, from which are obtained the crystals of naphthaline. When this deposit is raised to a temperature of 250° C. (482° Fahr.), the naphthaline and the in- definite oily substances are distilled away, and there is left anthracene, with some impurities. The impurities may be removed by means of the very light oils of petroleum, which dissolve them and leave the anthracene ; or by the light oils of coal-tar, which dissolve the anthra- cene and leave them. When anthracene has been sufficiently purified it is submitted to the action of oxidizing agents, and anthraquinone is obtained by precipitation as a resultant. By this direct process we have made a ternary body of our hydrocarbon, and have combined it with a proportion of oxygen which we can not increase by any further process of a direct character ; but the alizarine which we are seeking to get is richer in oxygen than anthraquinone. The second degree of oxidation has to be attained by an indirect process ; we bring it about by withdrawing some atoms of hydrogen from the molecule and sub- stituting for them molecules containing oxygen. The authors of the synthesis accomplished it in a process of two steps, by putting bromine in place of hydrogen and the elements of water in place of the bromine. But bromine is expensive, and so the manufacturers now make alizarine, not from a bromized but from a sulphureted anthraquinone. Of all the coloring substances derived from coal-tar, alizarine is the one which is now made in the greatest quantity. According to the report of M. Wtirtz, made in 1878, eight factories, two of which were very exten- sive, were then in full activity in Germany, two in Switzerland, one in England, and one in France, which last the proprietors had had the courage to establish in the very center of the madder-raising district. The quantity of alizarine then produced was estimated at 3,500 kilo- grammes, or nearly 9,000 pounds daily, and it has doubtless been since considerably increased.

Anthracene, the basis of the manufacture of alizarine, is relatively abundant in coal-tar, forming sometimes from seven to eight per cent of its mass. It has been observed that coal-tar is rich in anthracene in proportion as it is poor in toluene, and M. Berthelot has explained the fact by showing that toluene, decomposed by heat, produces anthra- cene ; hence the relative amount obtained of either is likely to vary according to the temperature-conditions of the distillation. The dif- ferences may also probably depend upon the character of the coal and of the matter first employed at the point of departure of all the opera- tions. But, as we have said, this point of departure is essentially un- known. All of our products have been obtained from a vegetable or organic, not from the primary mineral, carbon ; not from carbon either, but from compounds of carbon and hydrogen of a character which we have not yet been able to produce by synthesis of the primary min- eral elements, but which the sun stored up for us ages ago, working

�� � through the agency of organic growth. From a black and amorphous matter we have made to issue crystalline substances of every shade of color—reds, saffrons, greens, violets, and blues—alizarine, the same substance as tints the flowers of the madder, and that wonderful aniline, colorless as the ray of light before it has been resolved by the prism, but containing in posse, like the same ray, all the colors of the rainbow. What do we know of stone-coal, the origin of so many marvels and refractory to all analysis? Nothing, except that it has lived.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.

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