Page:Popular Science Monthly Volume 25.djvu/211

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ignorance, must now demonstrate the physical means and the exact locality of the fatal impress—perhaps found as a heart-obstruction, or a minute embolus deep in the labyrinth of the brain, to which some physiological clew may have led.

Aside from the inherent obscurity and difficulty connected with the subject of medicine, there remains as a heritage of by-gone ages an unwonted mystery associated with it, which should be more rapidly dispelled; and while the profession is making good progress in elaborating and writing its more exact laws, it is the duty of the intelligent laity to free themselves from the vestiges of mysticism, and seize upon the more prominent and available facts and principles which are their appropriate possession.


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COAL AND THE COAL-TAR COLORS.

By M. 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