194 Mr. M. M. Pattison Muir on Chemical Classification. 



of compounds whose molecular weights and densities are 

 known, and if we can establish a connexion, admitting of 

 exact expression, between these three quantities, then, when 

 we meet with a new compound which appears from general 

 analogies to belong to the group in question, the mere deter- 

 mination of its density may aid us materially in fixing the 

 molecular weight and, generally, in classifying the compound. 

 Further, Kopp's researches show that there is a connexion 

 between the structure of analogous compounds and their mo- 

 lecular or specific volumes. Determinations of the molecular 

 volumes of series of analogous compounds, if very carefully 

 made, must evidently be of the greatest assistance to the che- 

 mist who is attempting to reduce to some kind of order the 

 immense array of imperfectly examined compounds accumu- 

 lated by the industry of his fellow workers. In the course of 

 determinations of molecular volumes, various valuable data 

 bearing more or less directly upon the main question are also 

 accumulated. 



For these and other reasons chemists most anxiously expect 

 the publication of that series of researches upon which Thorpe 

 has been engaged for some years and which is now approach- 

 ing completion. 



The researches of Gladstone*, of Landoltf, and of others 

 most clearly indicate that a connexion exists between the che- 

 mical composition of compounds and their action upon light. 

 We have groups of terpenes most clearly marked off from each 

 other, on the one hand by difi'erences in molecular weight, on 

 the other by dififerences in refractive indices, in dispersion, in 

 sensitiveness, and so on. We have other bodies, often having 

 the same molecular weights and the same empirical formulas, 

 but differentiated by the action exerted by them on a ray of 

 polarized light. Can we doubt that structure and action on 

 light are connected together in some way ? 



Lastly, I would refer to the connexion which undoubtedly 

 exists between chemical composition and the thermal pheno- 

 mena exhibited during the formation and decomposition of 

 groups of compounds. When soluble hydroxides are neutra- 

 lized by addition of sulphuric, hydrochloric, or nitric acid, the 

 amount of heat evolved is almost the same in each case. W^hen 

 mixtures of acids are presented, in the proportions expressed 

 by the molecular weights, to the same hydroxide, the thermal 

 disturbances Avhich ensue show that the hydroxide divides itself 

 between the two acids in a definite manner. From a study of 

 such thermochemical reactions, we are able to draw conclu- 



* Chem. Soc. Joiirn. [2] vol. viii. pp. 101, 147, [2] vol. x. p. 1 &c. 

 t Pogg- Ann. cxxiii. p. 608. 



