262 PROCEEDIiSrGS OF THE AMERICAN ACADEMY. 



thermodynamic scale we possess the ideal measure of temperature. 

 So indeed the idea of escaping tendency, although not distinctly formu- 

 lated, has been tacitly recognized and used, and as a measure of the 

 escaping tendency the vapor pressure has been employed. Now if all 

 vapors obeyed the laws of a perfect gas, probably no better measure 

 could be found. But this is never strictly the case, and the more the 

 vapor departs from the ideal condition the more unsatisfactory is the 

 vapor pressure as a measure of escaping tendency. By introducing a 

 more satisfactory measure of escaping tendency we may gain advan- 

 tages similar to those which resulted from the substitution of the 

 absolute scale of temperature for the mercury scale. 



Such a measure of the escaping tendency I have described and used 

 in a previous paper.^ It was called the fugacity, and so defined that 

 the fugacity of a perfect gas is equal to its pressure. The fugacity of 

 an imperfect gas differs, however, from the gas pressure by an amount 

 which is greater, the more the gas deviates from the gas law. 



The idea of fugacity is thus evolved from the use of vapor pressure 

 as a measure of escaping tendency. When a substance is in equilib- 

 rium with its vapor, the fugacity, in order to fulfil the laws of escap- 

 ing tendency, must be the same iii both. The fugacity of a substance 

 is therefore equal to its vapor pressure if the vapor behaves like a per- 

 fect gas. Speaking in terms not very precise, we may say that the 

 fugacity of a substance is equal to the vapor pressure that the substance 

 would have if its vapor were a perfect gas. It has been shown in the 

 preceding paper that for a given substance in a given state the fugacity 

 is a definite property of which the numerical value can in most cases 

 be readily determined, and which is well suited to serve as an exact 

 measure of the escaping tendency. 



In many thermodynamic equations it is convenient to use concentra- 

 tions instead of pressures. Likewise we shall find it desirable to intro- 

 duce besides the fugacity, which has the dimensions of pressure, another 

 quantity which has the dimensions of concentration. This quantity 

 we will call the activity, and denote by the symbol ^. The activity will 

 be defined in terms of the fugacity, i/', by the following equation, 



where R is the gas constant and T is the absolute temperature. Since 

 the fugacity of a perfect gas is equal to its pressure, it is obvious that 



3 The Law of Pliysico-Chemical Change. Zeit. phys. Chem., 38, 205 (1901); 

 These Proceedings, 37, 49 (1901). 



