LEWIS. — A NEW SYSTEM OF THERMODYNAMIC CHEMISTRY. 263 



the activity of a perfect gas is equal to its concentration. If 11 has its 

 ordinary value, £ will be given in mols per liter. 



Both the fugacity and the activity are well adapted to serve as 

 measures of the escaping tendency. Indeed, for isothermal changes 

 the equations in which the two quantities enter are as a rule identical. 

 However, since the equations for the change of fugacity with the tem- 

 perature are a little less simple than those of the activity, we shall 

 choose the latter quantity for our present purpose. We shall start 

 with a simple definition of the activity, and proceed to show that the 

 change of the activity with the variables which determine the state of 

 the system may be expressed by a series of exact equations which are 

 of the same form as many of the familiar approximate equations for 

 vapor pressure, solubility, etc. 



On account of the large scope of this undertaking our consideration 

 will be limited to those systems which are completely determined by 

 the temperature, the pressure, and the composition of the various 

 phases. How the work may be extended to include other variables, 

 such as surface tension, has been indicated in the preceding paper. 



Fundamental Laws and Assumptions. 



The following work will be based on the two laws of thermodynamics 

 and upon the law that every gas and every solution as the concentra- 

 tion diminishes approaches as a limit the perfect gas and the per- 

 fect solution. Besides these we shall use the following definitions of 

 the activity. 



When the activity of a substance is the same in two phases, that 

 substance will not of itself pass from one phase to the other. 



When the activity of a substance is greater in one phase than in 

 another, the substance will pass from the one phase to the other, when 

 they are brought together. 



The activity of a perfect gas is equal to its concentration. 



The activity of the solute in a perfect solution, at constant tempera- 

 ture and pressure, is proportional to its concentration. 



We shall see that these statements suffice to define the activity of 

 a substance in any state, and except in unusual cases enable us to 

 calculate its numerical value. 



No further assumptions are necessary, but since our aim will be to 

 lay stress rather on the exactness of the results obtained than upon the 

 mathematical rigor of the method by which they are demonstrated, we 

 shall adopt as working aids the following assumptions : 



(1) For every molecular species we will assume that an ideal solvent 



