310 PHYSICAL CHEMISTRY 



second law of thermodynamics. This principle may be formulated 

 in different ways. For my purpose let me say that it limits the pos- 

 sibility of natural processes to the occurrence of those in which a dif- 

 ference of intensity is diminished. If there is a difference of pressure 

 in two parts of a gas, a movement will occur producing equality; if 

 there is a difference of temperature, heat will be transported so as to 

 produce equality once more. It is curious that such simple necessities, 

 which we all feel as such, can be converted into far-reaching sharply 

 formulated equations, as was done by Carnot and Clausius. These 

 principles were first applied in chemistry by Horstmann. Then, by 

 successive application to chemical problems by Massieu, Gibbs, Helm- 

 holtz,and others, was won a system of relations touching the problem 

 of affinity, to which I can give only brief attention: 



(1) Affinity may be defined as the maximum quantity of work that 

 a chemical change can produce. Equilibrium ensues when this quan- 

 tity is zero. 



(2) The mass-law can be obtained in a well-founded and somewhat 

 modified form, restricted to dilute gases and solutions. 



(3) The Thomson-Berthelot principle assumes a modified form in 

 the rule that a fall of temperature induces the formation of that which 

 develops heat. It is, for instance, in accordance with this rule that 

 at ordinary temperatures water is stable in comparison with detonat- 

 ing gas, and that at high temperatures this relation is reversed, as it 

 was found by Deville to be. 



(4) Lastly, we have the phase rule, indicating, for example, in what 

 cases chemical phenomena will be comparable with melting and freez- 

 ing, and in what cases they will be comparable with evaporation and 

 condensation. 



Most curious of all, w r e can treat problems of affinity in an abso- 

 lutely trustworthy way, so that our calculations furnish a check upon 

 experiment, without admitting anything concerning the nature of 

 affinity or of the matter wherein the affinity is supposed to reside. 



