478 PROCEEDINGS OP THE AMERICAN ACADEMY. 



d\n 



z^H z\^'y ... - Q 



dT RT' 



It will be seen at once that the sum of the reacting tendencies of the 

 initial substances In z^\ In 2:'i"'i, etc., agree in sign with Q ; that is, a 

 reaction which absorbs heat will have a greater tendency to take place as 

 the temperature rises. 



Besides comprehending the partial pressures within a single phase, and 

 the " physico-chemical potentials " of the various substances which are 

 in it, the pressure equations may be applied to the relations between two 

 2)hases ; since the equation from which they were derived is true for ideal 

 heterogeneous equilibrium. We may then introduce into the equation 

 phase-pressures, such as vapor or solution tensions, which are exerted 

 in opposition to gas or osmotic pressures respectively. Such phase- 

 pressures will be represented, for the sake of convenience, by the usual 

 designation P. 



A few simple physical examples may now be cited, to show that the 

 point of view is not inconsistent with the facts in these cases. For 

 example, in the case of an evaporating liquid, we may say that the force 

 concerned is the vapor tension, which therefore takes the place of z-^. It 

 is placed in the denominator because the process absorbs heat. There 

 are no opposing forces except the constant gas pressure against which 

 the evaporation proceeds \ hence the equilibrium equation becomes 



lo^=0, 



— an obvious truth ; and the reactiou-metatherm reduces at once to the 

 equation of Clausius. 



A more complicated case is the evaporation of water from a crystal- 

 lized salt ; but the treatment is equally simple. The water h'as a certain 

 reacting tendency or driving energy which manifests itself as a definite 

 pressure at each temperature. Precisely the same explanation applies 

 to calcic carbonate, or any other similar case. The simplicity of the 

 conception of this reacting tendency seems to commend itself as an 

 improvement over the usual involved treatment, since it does not demand 

 that all the solid should be vaporized in order to effect the change. 



A still more complicated case is the evaporation of ammonic sulphy- 

 drate. Here we have to consider the physical tendency of the evaporat- 

 ing substance, which is balanced by the pressure of the ammonic sulphy- 



