PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 5 DECEMBER 22, 1919 Number 12 
TEE CHANGE OF MOLECULAR KINETIC ENERGY INTO 
MOLECULAR POTENTIAL ENERGY: THE ENTROPY 
PRINCIPLE AND MOLECULAR ASSOCIATION 
By William D. Harkins 
Kent Chemical Laboratory, University of Chicago 
Communicated by J. Stieglitz, September 16, 1919 
My work on the orientation of molecules in the surfaces of liquids 
(Langmuir has also worked on orientation) has led to the recognition 
of a remarkable new principle or law concerning the change of molec- 
ular kinetic energy into molecular potential energy. None of the 
kinetic relations already found in the very simple case where gases 
alone are involved, have been found to be as exact as the laws of 
thermodynamics, since by their very nature such relations are affected 
by many extraneous compKcating factors. It need not then be surpris- 
ing if similar relations involving the much more complicated and hitherto 
obscure kinetic phenomena of the liquid and the solid states should 
prove to be approximate rather than exact. Thus it is well known that 
Raoult's law concerning the kinetics of the vaporization of the various 
components in a solution, is exact only in an extremely limited range, 
that is when the components are practically alike with respect to cohesion^ 
or with respect to the electromagnetic fields surrounding their molecules.^ 
It is, therefore, somewhat startling to find that my new relation or law 
is, in one of its forms, much more generally applicable than Raoult's 
law, at least if the data at present accepted in this connection are as 
exact as they are supposed to be. 
The new principle or law will first be stated in one of its special forms 
as follows : Whenever a molecule moves from the interior of a liquid into the 
surface in such a way as to form a new surface, the average amount of its 
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