54 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Instead of using this equation we may introduce here a quantity with the 

 aid of which it is possible to substitute for approximate equations of the 

 type of (7) other entirely exact equations of the same form. This 

 quantity is one whose utihty I have shown in a recent paper.* It 

 may be well to repeat and amplify the definition there given. 



II. 



FUGACITY. 



If any phase containing a given molecular species is brought in contact 

 with any other phase not containing that species, a certain quantity will 

 pass from the first phase to the second. Every molecular species may 

 be considered, therefore, to have a tendency to escape from the phase in 

 which it is. In order to express this tendency quantitatively for any 

 particular state, an infinite number of quantities could be used, such, for 

 example, as the thermodynamic potential of the species, its vapor pres- 

 sure, its solubility in water, etc. The quantity which we shall choose is 

 one which seems at first sight more abstruse than any of these, but is in 

 fact simpler, more general, and easier to manipulate. It will be called 

 the fugacity,t represented by the symbol ij/ and defined by the following 

 conditions : — 



1. The fugacity of a molecular sjiecies is the same in two phases when 

 these phases are in equilibrium as regards the distribution of that species. 



2. The fugacity of a gas approaches the gas pressure as a limiting 

 value if the gas is indefinitely rarefied. In other words, the escaping 

 tendency of a perfect gas is equal to its gas pressure. 



That these two conditions are sufficient to define a property of every 

 substance which is not a mathematical, fictitious quantity, but a real 

 physical qu.antity, capable of experimental determination in every case, 

 must now be shown. It is obvious from the above conditions that in any 

 case where our present methods of measurement are unable to show a 

 deviation of the vapor of a substance from the gas law then the vapor 

 pressure is the nearest approximation to the fugacity. In all cases the 

 vapor pressure is an approximation to the fugacity, the approximation 

 being nearer the nearer the vapor is to a perfect gas. When the 



* Proc. Amer. Acad., XXXVI. 145 (1900) ; Zeit. Phys. Chem., XXXV. 343 (1900). 



t In the earlier paper this quantity was called the escaping tendency and repre- 

 sented by the same symbol. For the sake of brevity I have chosen to substitute 

 the word " fugacity " for " escaping tendency " without the slightest change in the 

 meaning of the function. 



