THE LAW OF PHYSICO-CHEMICAL CHANGE. 

 By Gilbert Newton Lewis. 



Received April G, 1901. Presented by T. W. Richards, April 10, 1901. 



Introduction. 



The many-sided application of thermodynamics to physical chemistry 

 in recent years has led to a maze of mathematical expressions which is 

 bewildering to the beginner and confusing even to the initiated. The 

 great majority of these physico-chemical formula; arc based not only 

 upon the two laws of thermodynamics but also upon some empirical law 

 or approximation, and are as a rule not rigorously true, but are useful in 

 so far as the system considered does not deviate too widely from certain 

 ideal conditions. The difficulty of treating mathematically equations 

 which are not strictly exact is probably the chief reason for the con- 

 tinued separate existence of the large number of formulae which, though 

 not identical, are tantalizingly similar in form. It seemed probable that 

 if the present formulae could in any way be replaced by rigorously exact 

 ones, without sacrificing concreteness or immediate applicability, then 

 these exact equations might be so systematized that one might serve 

 where a number of isolated equations are now in use, with a great gain 

 in simplification. With this object in view the present investigation has 

 been carried on, and with the unexpected success of finding a single law 

 which is simple, exact, general enough to comprise in itself many laws 

 and yet concrete enough to be immediately applicable to specific cases. 

 The following development will be based upon four laws of nature and 

 upon no other hypothesis or assumption of any kind. These laws are 

 the following : — 



1. The first law of thermodynamics. 



2. The second law of thermodynamics. 



3. Every gas, when rarefied indefinitely, approaches a limiting condi- 

 tion in which 



Pv = RT, (1) 



if P represents pressure; v, molecular volume; R, the gas constant; T, 

 the absolute temperature. 



VOL. XXXVII. — 4 



