_ 2 K2EEDIHGS OF THE AMERICAS ACADEMY. 



thermodynamic Bcale we poa ees the ideal measure of temperature. 

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

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

 endency the vapor pressure has been employed Now if all 

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

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



vap ir departs from the ideal condition the more unsatisfactory is the 

 vapor pressure as a measure of escaping tendency. By introducin 

 more satisfactory measure of escaping tendency we may gain advan- 

 tages similar to those which resulted from the substitution of the 



lolute scale of temperature for the mercury scale. 



Such a measure of the escaping tendency I have described and use 1 

 in a previous paper. 3 It was called the fugacity, and so denned thai 

 the fugacity o I' 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 in 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 thi 

 fugacity of a substana is • qual to the vapor pressure that the substa 



ild have if its vapor were a ! gas. It has been shown in the 



preceding paper that for a given sul i in a given state the fugs 



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

 be readily determined, and which is well suited to serve as an e.\ 

 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, anothel 

 quantity which has the dimensions of concentration. This quantity 

 we will call the activity, and denote by the Bymbol & The activity will 

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



6 III 



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 Phyaico-Cheraical Change. Zeit. phys. Ctaem., 38, 205 (1901)J 

 These Proceedings, 37, 19 (1901). 



