Page:The American Cyclopædia (1879) Volume IV.djvu/377

CHEMISTRY 369 amyl alcohol, which suggested the idea that the term alcohol must be regarded not merely in a specific but in a generic sense. Similar analo- gies were soon observed among other organic compounds, and it gradually became evident that very many organic substances could be classified together in homologous series, the composition of each member of which differs from that of the others, either by a certain number of equivalents of carbon and hydro- gen, or by some simple multiple of this num- ber ; while the chemical properties of the sev- eral members of the series are entirely analo- gous, differing only in degree in direct propor- tion to the amount of carbon and hydrogen which they contain ; whence, the general prop- erties and relations of any one member of a series being known, those of any other of its members may be directly inferred. Remark- able relations between the physical properties of the different members of the same homolo- gous series have also been noticed. Thus, H. Kopp has observed that the point at which the several members of a series boil increases about 19 centigrade for every two equivalents of carbon (old value) plus two of hydrogen which they contain more than the first mem- ber of the series. It is admitted by Kopp that the double atom of carbon elevates the boiling point 29 C., while the double atom of hydrogen diminishes it 10 0. The fusing point also of the members of several series presents analogous relations, while Kopp has shown by a number of most important investi- gations upon atomic or specific volumes (by which terms are understood the relative spaces occupied by the atoms or equivalents of bodies, being the quotients obtained by dividing their equivalent weights by their specific gravities), that the atomic volumes of many homologous compounds differ from each other by a con- stant quantity, proportional to the number of double at3ms of carbon and hydrogen which they contain. The credit of the first clear perception of this wonderful system of homol- ogism is due to Gerhardt (1816-'56). No system has done more to advance organic chemistry than this, while no one of his con- temporaries has surpassed its gifted and labo- rious author. Of the chemists who especially devoted themselves to the advancement of the doctrine of substitution, no one can be com- pared with Laurent (1807-'53). Denouncing compound radicals as purely hypothetical bodies, he endeavored to substitute for them his own theoretical nuclei. In itself this theory has exerted comparatively little influence. In the hands of Laurent, however, it led to the discovery of an immense number of new com- pounds, the very naming of which required that a special nomenclature should be framed. Few works of recent times have displayed greater originality than his Methode de chimie. The comparative harmony now existing among chemists in their views regarding organic sub- stances was in great measure brought about by the investigations of the compound am- monias and ammoniums by Wurtz of Paris and Hofmann of London. Kolbe of Marburg and Frankland of Manchester had indeed isola- ted bodies of the same composition as methyl, ethyl, and other radicals, but their properties were less active than chemists had expected, and their identity with the true radicals was not at once admitted. The question was, how- ever, settled by Hofmann's discovery that one or all of the equivalents of hydrogen in ammonia may be replaced by an equal number of equiva- lents of one radical possessing basic properties, like ethyl or methyl, or by several such radi- cals, so that four different radicals may be pres- ent in one equivalent of ammonium. Chemists were now forced to admit that the radicals entered into combination precisely as if they were elements, replacing hydrogen equivalent for equivalent. In like manner Gerhardt dem- onstrated that one or more of the three equivalents of hydrogen in ammonia may be replaced by a compound radical containing oxygen, as benzoyl ; thus proving the correct- ness of the original benzoyl theory, which had suffered so many attacks. The extension given by these researches to the idea attaching to the term ammonia was immense. It became at once generic, including innumerable bodies, all possessing to a greater or less extent the char- acteristic properties of the original ammonia. Among the most interesting types of organic compounds are those in which two bodies of analogous nature are united together in such a manner that the properties of the compound resemble in kind those of one of its ingredients, the original properties of the latter being mod- ified only in degree. Such compounds are call- ed copulate or conjugate, and may be either acids, bases, or radicals. Many of the copulate acids were studied by Berzelius and his imme- diate followers, while the conjugate radicals have been especially investigated by Frank- land, Lowig of Breslau, and others, among whom Prof. Wolcott Gibbs of New York and Dr. F. A. Genth of Philadelphia deserve special mention The first investigation of this sub- ject, and one of the finest in the records of chemical science, was that by Bunsen of Heidel- berg on kakodyl, a radical composed of two equivalents of methyl united with one of arse- nic. Of deep interest is the success which has recently attended attempts to prepare organic compounds by combining others of more simple composition. For a long time urea was the only compound which could be thus prepared, but within the last few years many such have been obtained. In this department, the ex- periments of Berthelot of Paris are most im- portant. By heating carbonic oxide gas with hydrate of potash in a sealed vessel, he has ob- tained formiate of potash, from which formic acid may be readily prepared in a free state. By agitating olefiant gas (one of the compo- nents of ordinary coal gas) with oil of vitriol, a compound is formed from which, on addition