302 PRINCIPLES OF CHEMISTRY 



plete idea of the dependence of the properties of a compound on the 

 properties of the component substances, it is further necessary to know 

 the quantity of heat which is developed in the formation of the com- 

 pound. If this quantity be large as, for example in the formation of 

 water then the amount of energy in the resultant compound wilt be 

 considerably less than the energy of the elements entering into its com- 

 position ; whilst, 011 the contrary, if the amount of heat evolved in the 

 formation of a compound be small, or if there even be an absorption 

 of heat, as in the formation of nitrous oxide, then the energy of the 

 elements is not destroyed or is only altered to a slight extent : hence, 

 notwithstanding the contraction (compression) involved in its forma- 

 tion, nitrous oxide supports combustion. 



The preceding laws were deduced from purely experimental and 

 empirical data, and as such evoke further consequences, as the law of 

 multiple proportions evoked the atomic theory and the law of equiva- 

 lents (Chapter IV.). In point of view of the atomic conception of the 

 constitution of substances, the question naturally arises as to what 

 then are the relative volumes proper to those physically-indivisible 

 molecules which chemically react on each other and consist of the 

 atoms of elements. The simplest possible hypothesis in this respect 

 would be that the volumes of the molecules of substances are equal ; or, 

 what is the same thing, to suppose that equal volumes of vapours and 

 gases contain an equal number of molecules. This proposition was 

 first enunciated by the Italian savant, Avogadro, in 1810, It was 

 also admitted by the French physico-mathematician, Ampere (1815), 

 for the sake of simplifying all kinds of physico-mathematical concep- 

 tions respecting gases. But Avogadro and Ampere's propositions were 

 not generally received in science until Gerhardt in the forties had 

 applied them to the generalisation of chemical reactions, and had 

 demonstrated by aid of a series of phenomena that the reactions of 

 substances are actually accomplished with the greatest simplicity, and 

 before all, that such reactions take place between those quantities of 

 substances which occupy equal volumes, and until he had stated the 

 hypothesis in an exact manner and deduced the consequences that 

 necessarily flow from it. Following Gerhardt, Clausius, in the fifties, 

 placed the hypothesis of the equality of the number of molecules in 

 equal volumes of gases and vapours on the basis of the kinetic theory 

 of gases. Since then the hypothesis of Avogadro and Gerhardt has 

 become the basis of contemporary physical, mechanical, and chemical 

 conceptions ; the consequences arising from it have often been subject 

 to doubt, but in the end have been verified by the most diverse methods, 

 and now, when all efforts to refute those consequences have proved 



