54 
CHEMISTRY: HARKINS AND EWING Proc. N. A. S. 
the total energy changes in the film, which include both the solid carbon 
and the liquid of the interface. This heat is entirely analogous to that 
developed when a body falls from a height above the earth to its surface, 
when the mutual potential energy of the body and the earth are converted 
into molecular kinetic energy, or heat. In a similar way, when the sur- 
face of the liquid and of the carbon fall together, their mutual potential 
energy of molecular attraction, due to electromagnetic (largely electrical) 
forces, is converted into molecular kinetic energy, which is heat. 
This theory of the heat of adsorption has been developed by Harkins, 
but has not been published, although the fundamental equations upon 
which it is based have been published in these ProcBKdings. The final 
equations, (1) and (4), as given above, were not, however, presented. 
The experimental work on interfaces as carried out in this laboratory 
indicates that the 7^ of equation (1) is very large, and that most of the 
heat which appears is due to the fact that the interfacial free energ}^ carbon- 
liquid is very much smaller than the free surface energy of the solid car- 
bon. Equation (3) shows that the heat of adsorption of the liquid is equal 
to the energy of adhesion between the liquid and the carbon, minus the 
total surface energy of the liquid. In the case of water this total surface 
energy is moderately large, so the heat of spreading of the liquid is con- 
siderably less than the adhesional energy. 
If further data agree with those already found in indicating that the 
differences in the volumes of various liquids absorbed by activated cocoa- 
nut shell and other highly activated charcoals are due to a compression 
by the forces of adhesion of the order of twenty thousand atmospheres 
as the average pressure in a film of 4 X 10 ~^ cm. in thickness, which would 
indicate a much higher pressure for the first layer of atoms, then the re- 
sults presented in this paper will be the first direct evidence of the exis- 
tence of such high forces between solids and liquids, or in liquids. Thus, 
while the theories of Laplace and van der Waals indicate an internal 
pressure of 10,000 atmospheres in water, and usually from 200 to 4,000 
atmospheres in an organic liquid, it has not been found possible to demon- 
strate experimentally forces greater than 34 atmospheres with water at 
24.4° C, 39.5 atmospheres for ethyl alcohol at 22.5, and 72 atmospheres 
for ether at 17.7 ^.^ 
The area of the surface in 1 gram of Miss Homfray's charcoal was 1.31 X 
10^ sq. cm. per gram of charcoal, and in Titoff's charcoal it was 8 X 10^, 
according to estimates made by Williams. ^ Lamb, Wilson, and Chaney^*^ 
have estimated the surface in a gas-mask charcoal as lO'^ sq. cm. per gram, 
or almost the same as in Titoff's charcoal. In this laboratory Mr. Amando 
Clemente has been working on the adsorption of vapors at low pressures, 
and his work seems to indicate an area very well in agreement with that 
of Lamb, Wilson, and Chaney, but experiments are still incomplete so the 
final value is uncertain. His work indicates that the adsorptive capacity 
