LEWIS. — A NEW SYSTEM OF THERMODYNAMIC CHEMISTRY. 261 



same temperature as each other. (2) If a body A has a higher tem- 

 perature than the body B, it has a higher temperature than any other 

 body of the same temperature as B. 



These are not self-evident truths, but empirical laws. If they did 

 not exist, the idea of temperature would lose all value. Temperature 

 determines the distribution of energy in a system, and we may regard 

 the temperature of a body as a measure of the tendency of its internal 

 heat to escape into other bodies. 



There are in chemistry two laws which are in every way analogous 

 to the laws of thermal exchange. If a system is composed of several 

 parts, A, B, C, D, containing a given molecular species, X, the two fun- 

 damental laws concerning the distribution of X throughout the system 

 are the following: (1) If when the phases A and B are brought to- 

 gether there is no transfer of X from one to the other, and if the same 

 is true of A and C, then when B and C are brought together there will 

 likewise be no transfer of X. (2) If X passes from the phase D to the 

 phase A when they are brought together, then it will also pass from D 

 to B, or to C, or to any phase which is in equilibrium with A as regards 

 the distribution of X. It is obvious that these two laws follow directly 

 from the fundamental laws of thermodynamics, for if they were not 

 true a system could be constructed capable of perpetual motion. 



The escaping tendency of a given molecular species in a given state 

 is therefore analogous to temperature, and the two laws of escaping tend- 

 ency are as follows: If the escaping tendency of a given molecular 

 species, X, is the same in two phases, then X will not of itself pass from 

 one phase to the other. If the escaping tendency of X is greater in 

 one phase, it will pass from this phase into the other, when the two are 

 brought together. 



Let us illustrate the meaning of the escaping tendency by an exam- 

 ple. When in a pure liquid a small quantity of some other substance 

 is dissolved, the vapor pressure of the liquid is diminished, its freezing 

 point is lowered, its boiling point is raised, its solubility in another sol- 

 vent is diminished. 2 All these laws are comprised in the simple state- 

 ment, that the escaping tendency of the solvent is diminished by the 

 addition of the solute. 



The idea of temperature was understood long beiore any suitable 

 measure of temperature was found. Then the mercury thermometer 

 was invented, later the gas thermometer, and finally in the absolute 



2 So also the tendency of the liquid to take part in any chemical reaction is 

 diminished, hut until a later section of this paper our discussion will he limited 

 to processes in which a given molecular species passes from one phase to another 

 without any other change. 



