II. KELATIONS BETWEEN CHEMICAL 

 PROPERTIES AND COMPOSITION 



As in dealing with physical properties in regard to 

 composition, so here there is a preliminary question to be 

 answered, how the property in question is to be represented 

 numerically in a manner as independent of other circum- 

 stances as possible. How far this can be answered in the 

 case of the physical properties has appeared in the fore- 

 going ; the conditions are less favourable here, on account 

 of the more complex character of chemical properties. 

 Thus the way in which temperature affects volume is in 

 many cases, such as the gases, known : the way in which 

 it affects velocity of reaction has not yet been brought 

 satisfactorily under a general view. Accordingly the rela- 

 tions with which we shall have to do now are throughout 

 of an empirical character, and often merely qualitative. 

 The arrangement must be made with a view to this, and 

 refers to the chief rules which are of general application in 

 this region of the phenomena considered l . 



1 Note here a very important remark of Le Chatelier, 'Recherches 

 experimentales et theoriques sur les equilibres chimiques/ Annales des 

 Mines, 1888, p. 193. According to this the dissociation pressure and 

 other data referring to chemical equilibrium are associated with the 



q 



Deprez-Trouton law. Thus for calcium carbonate 23.4 at the tem- 

 perature for which the dissociation pressure is one atmosphere (q is the 

 molecular heat of formation of CaC0 3 from lime and carbon dioxide). 

 Unfortunately there are not at present data enough for an extended 

 application, such as might be sought in the first place amongst salt 

 hydrates. 



