422 SCIENCE PROGRESS. 



actions taking place at low temperatures are alone con- 

 sidered, there are so many exceptions to the rule that no 

 general validity can be claimed for it. The chief reason 

 for the comparative want of success of thermochemistry in 

 the development of chemical theory is to be found in its 

 exclusive adherence to the first law of thermodynamics, the 

 second being altogether neglected. Of late years, however, 

 this has been largely remedied, so that we are now in a 

 better position to judge of the relation of heat to chemical 

 energy. 



The rate at which a chemical reaction takes place has 

 also been proposed as a measure of the forces at work in 

 producing the action, but this also is unsatisfactory. Sodium 

 and chlorine are generally supposed to have a powerful 

 affinity for each other, but if both are dry they will not 

 unite at the ordinary temperature. Sodium when added to 

 alcohol under ordinary conditions acts on it most vigorously, 

 especially if the alcohol contain water. Pictet, however, 

 has shown that at — 8o° sodium is without action on 

 aqueous alcohol. The actions then in these cases must go 

 on with such extreme slowness as to be imperceptible, yet 

 we attribute to the reacting substances considerable chemical 

 attraction for each other. 



There is, as has been said above, one class of chemical 

 reactions which is of great service in indicating- the direction 

 in which we are to seek for an explanation of chemical 

 affinity, which affords us a practical measure of relative 

 affinities, and which indicates the relation of these both 

 to velocities of reaction and to energy in general ; the 

 class, namely, of balanced actions, and it is such an action 

 that Berthollet has under consideration when he speaks of 

 a base being divided between two acids possessing different 

 affinities for it. We have each of the acids acting on the 

 salt of the other acid at a certain rate, and the point where 

 the one action balances the other is determined by the 

 velocities of the opposed reactions, and by the quantities of 

 the reacting substances. Guldberg and Waage showed 

 experimentally that the action of each substance is pro- 

 portional to its active mass, i.e., to its amount divided by 



