262 THE POPULAR SCIENCE MONTHLY. 



to Prof. T. E. Thorpe, from whose memoir in Nature we derive 

 most of the material of this sketch, he extends Kopp's generaliza- 

 tions, and traces the specific volumes of substances through vari- 

 ous phases of chemical changes. In a paper on the thermal ex- 

 pansion of liquids above their boiling-points, he showed that the 

 empirical expressions given by Kopp, Pierre, and others are equally 

 applicable to much higher temperatures, and that the expansion- 

 coefficient gradually increases with the diminution in molecular 

 cohesion of the liquid, until, in the case of some liquids, it becomes 

 even greater than that of the gas. In 1883 he contributed to the 

 English Chemical Society a paper giving a simple general expres- 

 sion for the expansion of liquids under constant pressure between 

 zero and their boiling-points — a formula analogous to that which 

 expresses Gay-Lussac's law of the uniformity of expansion of 

 gases ; but which, like Gay-Lussac's law, however correct in the- 

 ory, is subject to deviations in application. These deviations 

 were shown to be related to the molecular weights of the gases. 



Researches in thermal chemistry, made in 1882, showed him 

 that the data obtained by Berthelot, Thomson, and others, regard- 

 ing the " heats of formation " of hydrocarbons, stood in need of 

 correction, because allowance had not been made for the physical 

 changes involving absorption or evolution of heat which accom- 

 pany the chemical changes considered ; and he gave a table giving 

 the heats of formation from marsh-gas, carbon monoxide, and 

 carbon dioxide, of a series of hydrocarbons, for chemical reac- 

 tions that actually occurred, while the reactions given by Ber- 

 thelot and others were not realized in practice. 



In the investigation of solutions, Mendeleef propounded in 

 1884 the law that in solutions of salts the densities increase with 

 the molecular weights ; but if we take, instead of the molecular 

 weights, the weights of their equivalents or those of the equiva- 

 lents of metals, the regularity of increase disappears ; and, though 

 his research was not yet finished, he submitted an equation as 

 preliminary to ulterior results promising to give a more general 

 formula. The results of the determination of the specific gravity 

 of aqueous ~ solutions of alcohol were applied, according to Prof. 

 Thorpe's memoir, toward the elucidation of a theory of solution 

 in which Dalton's doctrine of the atomic constitution of matter 

 could be reconciled with modern views concerning dissociation 

 and the dynamical equilibrium of molecules. "According to 

 Mendeleef, solutions are to be regarded as strictly definite atomic 

 chemical combinations at temperatures higher than their dissoci- 

 ation temperature ; and, just as definite chemical substances may 

 be either formed or decomposed at temperatures which are higher 

 than those at which dissociation commences, so we may have the 

 same phenomenon in solutions; at ordinary temperatures they 



