540 
CHEMISTRY: W. D, HARKINS 
kinetic energy which is converted into potential energy is equal to 144% of 
the mean translational kinetic energy of a gas molecule at the same tem- 
perature. This indicates that in general only the faster moving molecules 
possess sufScient kinetic energy to carry them into the surface. All 
known plane surfaces have a positive free surface energy, that is the 
molecular potential energy in a plane surface in which the above prin- 
ciple holds is always greater than 144% of the mean kinetic energy of 
its molecules. 
Application to the theory of surfaces. — As has been indicated in the last 
paragraph the molecules in a surface possess potential energy by virtue 
of their position. When a new surface is formed the principle given 
above indicates that a definite portion of this potential energy results 
from a tranformation of the kinetic energy of molecular motion into the 
potential form, and that the amount of energy supplied in this form 
depends only upon the temperature; and is proportional to the tem- 
perature. The free energy of the surface is simply the difference between 
the total energy, which depends upon the structure of the surface, and the 
latent heat of the surface which is conditioned by the above law. It is easy 
to see why, on this basis, the surface tension or the free surface energy 
decreases with the temperature. The total surface energy is approxi- 
mately constant while the temperature is varied, provided the critical 
temperature is not too closely approached, which is the condition also 
for the apphcation of the new principle since the surface film thickens 
as the critical temperature is neared. Since the contribution of the 
kinetic energy of molecular motion to this total energy is propor- 
tional to the absolute temperature, the free surface energy must de- 
crease with the temperature. 
The entropy law. — According to Lunn^ heat has two measurable aspects, 
energy and entropy. The first law of thermodynamics relates to heat 
changes in which energy is, but entropy is not, conserved. The second 
law considers heat changes in which energy in the form of heat alone 
is not, but entropy is, conserved. The principle discovered by me has 
been stated above in terms of energy but it may be much more simply 
stated in terms of entropy as follows: Whenever a molecule moves from 
the interior of a liquid into its surface in such a way as to form a part of 
a new surface, the entropy of surface formation is not only independent of 
the nature of the molecule, but is also independent of the temperature. The 
numerical value of this entropy is 2.96 X 10~^^ ergs per degree per 
molecule. 
This entropy is not the ordinary thermodynamic entropy but is about 
