618 



NOMENCLATURE AND NOTATION, CHEMICAL. 



by weight. The spirit, then, of the chemical 

 theory now apparently being superseded, was 

 essentially that of recognizing ponderal equiv- 

 alencies only, as found to exist between bodies 

 elementary and compound. But it will be 

 seen, as we proceed, that the modern chemical 

 theory has its basis in the conjoint admission 

 and determination of both ponderal and volu- 

 metric combining-ratios ; while, as scarcely less 

 essential, it incorporates at the same time two 

 other leading ideas recently developed that 

 of substitutions, or replacements, of certain ele- 

 ments or groups in compound bodies ; and that 

 of definite molecular types, few in number, 

 under which all elementary and all compound 

 bodies would appear naturally to fall. 



Determination of Weight and Volume Ra- 

 tios. It will be proper here to premise so much 

 of chemical theory as to intimate the princi- 

 ples which have been, down to the present 

 time, gradually matured, for determining the 

 weight and volume ratios in which elementary 

 (and of course also, compound) bodies enter 

 into combination. Obviously, the relative com- 

 bining weights can usually be determined with 

 comparative ease ; and the volumes also, when- 

 ever the bodies under examination are such as 

 have naturally, or can be brought into, the gase- 

 ous form, at temperatures within which the 

 experiments can conveniently be carried on. 

 In the mode in which the modern chemical 

 philosophy regards the constitution of element- 

 ary and compound bodies, and attempts the 

 determination of their combining weights and 

 volumes, an indispensable part is played by the 

 hypothesis early advanced by Ampere, namely, 

 that of all bodies in the gaseous state (pressure 

 and temperature being the same) equal volumes 

 contain an equal number of atoms, or of mole- 

 cules ; and to the application of this assumed 

 principle, as yet, but a small number of appar- 

 rent exceptions have been met with. 



Let it for the present be granted, that in vol- 

 umetric determinations it is the 2-atom or dou- 

 ble volume which should be dealt with, and 

 the following statement, from Ilofmaun's Mod- 

 ern Chemistry, will serve clearly to indicate 

 the general rule of both weight and volume 

 determinations : 



" It is by first obtaining the gaseous or vola- 

 tile compound formed by an element, either with 

 hydrogen itself, or, failing this, with some ele- 

 ment bearing thereto known weight and volume 

 ratios, and by then ascertaining its vapor-density 

 and composition, so as to learn how much of 

 the element under investigation exists in the 

 dilitral [double] volume of such compound, 

 that the atom-weights of the elements, whether 

 volatile or fixed, can be most safely and cer- 

 tainly determined." 



In a small proportion of instances, in which 

 these usual modes of proceeding are impracti- 

 cable, or give uncertain results, certain analo- 

 gies traceable between different classes of com- 

 pounds, or those relations constituting groups 

 among tbe elements themselves, may be brought 



to the aid of the chemist ; and a_so certain 

 physical principles, if applicable to the bodies 

 in question. Among the latter, is the law of 

 Dulong and Petit, -to which but few exceptions 

 (C, Si, and Bo, at least) appear to remain, viz., 

 that the products of the atomic weights of the 

 elements into their specific heats, give nearly a 

 constant quantity a law from which, in con- 

 nection with Ampere's hypothesis, Regnault 

 (1849) deduced the principle of thermic propor- 

 tional numbers, or thermal equivalents, of the 

 elements. Other principles of the like sort 

 and value, especially in case of compound 

 bodies, are that afforded in the law of homolo- 

 gous series, as those of the alcohols, ethers, hy- 

 drocarbons, etc., with certain relations, as the 

 regular gradation of boiling-points, pertaining 

 to them ; as well as also the principle of iso- 

 morphism, or similarity of crystalline forms, etc. 

 Glance at the Hise of the Type Theory. 

 M. Auguste Laurent, November, 1846, first dis- 

 tinctly intimated the idea of types among com- 

 pound bodies. Thus, he considered alcohol, and 

 also certain hydrated oxides, as comparable 

 with water an atom of hydrogen being in 

 the former replaced by one of ethyle (C 2 H 5 ), 

 and in the latter by one of a metal : thus, call- 

 ing water H 2 O, 



H.HO, (O,H 5 ).HO, K.HO. 



Water. Alcohol. Hydrate of Potash. 



Professor T. Sterry Hunt, a young American 

 chemist, began (1848-'50) the expansion of this 

 idea to its consequences, and toward the full 

 breadth of its applications ; while, later, through 

 the labors and writings of Williamson, Gerhardt, 

 Wurtz, Hofmann, Odling, and others, this work 

 has been well-nigh completed ; so that the doc- 

 trine of typical forms among compound bodies, 

 and of a natural grouping or classification of 

 bodies under such types, has become developed 

 into a system, based on true scientific grounds, 

 and warranted especially by the light it throws 

 on cases of substitution, on chemical reactions 

 generally, and on the results to be anticipated 

 in the way of experiment, in case of given com- 

 pound bodies. The history and details of the 

 type theory in chemistry may possibly be given 

 in a future volume : at present, it must suffice 

 barely to indicate the main results reached. 



The new theory admits generally a distinc- 

 tion between the atom (equivalent, or combin- 

 ing proportion) of any body, and the molecule 

 (definite aggregation of atoms), or form under 

 which the same body exists in the free state. 

 It regards the molecules of most simple, and 

 of many compound bodies, as composed re- 

 spectively of two atoms (Ampere) ; so that free 

 chlorine is C1C1, or chloride of chlorine, etc. 

 Under it, four leading types have been deter- 

 mined, representing respectively 1-, 2-, 3-, and 

 4-atom combinations of certain elements or 

 compounds with the single atom of certain 

 others ; those types being the following : 

 HC1, H 2 O,. H,N, HiO. 



Chlorhydrlc acid. Water. Ammonia. Marsh-gas. 



Owing to the fact of the successively increas- 



