Page:The American Cyclopædia (1879) Volume IX.djvu/446

 432 ISOMERISM written CJIuO,. They are however proxi- ' mately composed of Formic anhydride. .C 2 H 2 3 Oxide of ethyle C 4 H 10 O C,U,,0 4 Acetic anhydride. ,.C 4 Tr,O 3 Oxide of mctbyle. . .C a II 6 O O.H,.0 4 That such compounds really contain different proximate constituents is proved by the fact that they afford different products when de- composed under similar conditions. Hence they are regarded as distinct chemical sub- stances, and not as modifications of one and the same body. The different grouping of the elements of these compounds has been com- pared to that of letters in words like ate, eat, tea, &c., on the arrangement of which the meaning of the word is entirely dependent. All bodies which are thus isomeric with each other, all that have absolutely the same ulti- mate composition, must of course possess iden- tical equivalent weights. Such substances are often called metameric (Gr. //era. indicating change or alteration), in contradistinction to polymeric substances (Gr. iroWf, many), which are composed of similar elements united in the same relative proportion in each case, but in different absolute quantities; the equivalent weights in which these substances combine with other bodies being unlike. This distin- guishes them from members of the preceding class, in which both the relative and absolute number of equivalents are the same. Very many polymeric substances are known, whole series of organic compounds being formed of them. As an example, olefiant gas and cetene both contain 85 '7 per cent, of carbon and 14-3 per cent, of hydrogen. If nothing were known of their chemical comportment, the empirical formula CHj would be applicable to both ; but by studying their properties it has been found that 20 and 4H have united to form olefiant gas, the rational formula of which is therefore C 2 H4, and its combining equivalent 28 ; while 160 and 32H have united to form cetene, which has consequently the rational formula CuH 82, and the equivalent number 224. Be- tween these two bodies there are 14 others polymeric with them and with each other. Differences like this have been compared to those between words like ma, mamma, tar, tartar, &c., which contain the same letters' arranged in the same way, but in different quantities. The arrangement of the elements in polymeric substances is not however of necessity the same; thus, the ether of wood spirit is polymeric with common alcohol; yet the rational formula of the former is OH 4 O ; of the latter, OjHjO. Until a comparatively recent period it was the prevalent opinion among chemists that bodies of similar composi- tion must of necessity possess similar properties. Any observations tending to throw doubt upon the correctness of this belief were considered erroneous. Even the discovery, by Wohler and Liebig, that cyanic and fulminic acids are of like percentage composition although they pos- sess very different properties, was attributed to errors of observation, and generally discredit- ed. Faraday's investigation of several isome- ric hydrocarbons in 1825 first proved the fal- lacy of this supposed law. Its exceptions, be- ing now more carefully observed, were found to be very numerous. In 1830 Berzelius pro- posed that they should be classified as isomeric substances. It was however soon perceived that the doctrine of isomerism could not with propriety be employed to explain the cause of all the differences which had been observed ; least of all, to explain those which occur among the elements themselves bodies which, from his inability to decompose them, the chemist is forced to regard as simple. In 1840 Berzelius suggested that these peculiarities might depend upon some absolute difference of quality in the different varieties of a substance, and not upon any dissimilarity in the arrangement or number of its molecules. He proposed the term allo- tropism (Gr. cM.6TpoKos, of a different nature) to express this idea, which has ever since stead- ily gained favor, although directly opposed to the doctrine of the immutability of matter, one of the principal tenets on which the chemistry of the first half of the present century was based. Allotropism is of special interest from the fact that several of the most common and best known elements may occur in two or more allotropic states. Thus, pure charcoal (lamp- black), graphite, and the diamond are essential- ly identical chemical substances. The element phosphorus, as it commonly occurs, is a soft, waxy, yellowish white, exceedingly inflamma- ble, and very poisonous substance, with a strong odor and taste, luminous in the dark, and read- ily soluble in bisulphide of carbon. It may easily be transformed, however, into another allotropic state, in which it is of a dark red, near- ly black, color; is hard, brittle, and devoid of taste or smell, and, so far as is known, of poison- ous properties ; is not luminous, and is complete- ly insoluble in bisulphide of carbon. It differs moreover from ordinary phosphorus in spe- cific gravity, and entirely in its affinity for oth- er substances. Indeed, it is not known that it is itself combustible ; for it may be heated without undergoing change to about 500 F., at which temperature it is reconverted into or- dinary phosphorus. These two conditions of phosphorus are so utterly unlike in all their properties, excepting the weight of their equiv- alent, that were it not in the power of chemists to prove their identity by converting them one into the other, they would without hesitation be considered distinct elements. Similar in- stances occur among gases. For example, or- dinary oxygen gas may be converted into an allotropic modification called ozone, which pos- sesses properties entirely different from those of the original oxygen. Chlorine gas also, ac- cording to Prof. J. W. Draper of New York, after exposure to strong sunlight, possesses the power of combining with hydrogen even in the dark, and exhibits other properties unlike those