Chemistry and Physics. 381 



By differentiating the energy with respect to each of these 



quantities, each of which has a 



dated to that of the variable to 



which it corresponds. 



Thus, by < . we obtain 



>>i tilt.' fluid with it- sign reversed; by « I i tit ■ n_- 1 1 1 hating 



w.h respect to the entropy, we obtain the temperature on the 



inic scale; ami bv tlifhreutiating with respect to the 



mass of any one of the component substances, we obtain what 



Professor Gibbs calls the potential of that substance in the mass 



considered. 



As this conception of the potential of a substance in a given 

 homogeneous mass is a new one, and likelv to become very impor- 

 tant in the theory of chemistry, I shall give Professor Gibbs's 

 definition of it. 



- homogeneous mass we suppose an infinitesimal quan- 

 remaining homogeneous 

 unchanged, the increase 

 of the energy'of the mass, divided by'the mass of the substance 

 ad.h.,1, is the potential of that substance in the mass considered.' 



These n-\-2 new quantities, the pit-sure, the temperature, and 

 tlif -< potentials <.f the component siib>tances, form a class differ- 

 ln - in kind from the first set of variables. They are not quanti- 

 ties capable of combination b\ i 1 1 ti i » i r 1 i ' til. intensity 

 of certain physical properties of the substance. Thus the pressure 

 is the intensity of the tendency of the body to expand, the temper- 

 ature is the intensity of its tendency to par: 



potential of a n v component substance is the intensity with which it 

 tends to exp a its mass. _ 



We may therefore distinguish these two classes of variables by 

 calling the volume, the entropy, and the component masses the 

 '•[":i»',-,„u^ and the pressures, the temperature, and the poten- 

 tials the intensities of the system. 

 The problem before us may 



the whole or part of it pass i 



The criterion of equilibriu, 



Gibbs's words—' For the eqi 



also be expressed by saying that tor a 

 the state of the svst. 

 entropy, the variation of its energy shall either vanish or be pos 



Professor Gibbs has made a most important contribution 1 

 science by giving , ls a i ■ , fi " >t 1 a "'' f> ' 



any given plK. other phase K 



letter K) is positive, the phase A will not of itself pass into tl 



