Page:Popular Science Monthly Volume 25.djvu/221

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211
COAL AND THE COAL-TAR COLORS.

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 indefinite 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 anthracene 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 substituting 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. Würtz, made in 1878, eight factories, two of which were very extensive, 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 kilogrammes, 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 anthracene; hence the relative amount obtained of either is likely to vary according to the temperature-conditions of the distillation. The differences may also probably depend upon the character of the coal and of the matter first employed at the point of departure of all the operations. But, as we have said, this point of departure is essentially unknown. 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 mineral elements, but which the sun stored up for us ages ago, working