564 
CHEMISTRY: W. D. HARKINS 
Energy of adhesion. — The equation developed by me gives the total 
adhesional energy (E^), and is as follows: 
' = -AEs = (71 + /i) + (72 + I2) - (71,2 + h,2) (4) 
The energy of adhesion is equal to the total surface attraction, {ir") due 
to molecular attraction, integrated through the distance necessary to 
pull the two surfaces completely apart, or 
Ea = Itt" ds = E1 + E2- El, 2 (5) 
The only experimental results which have been obtained in connection 
with equation 4 are those of experiments by Dr. E. C. H. Davies and 
Mr. Y. C. Cheng as carried out under my direction. This seems to be 
the first work aside from that on the heat of adsorption, which bears 
very directly on the molecular attraction between unlike substances. 
Energy of surface cohesion. — When the two liquids become identical 
equation 4 reduces to the form 
Esc = -AEs = 2 (7 +0 (6) 
It is evident that the total energy of surface cohesion is equal to the total 
tensile energy (Erp), or 
Et = -AEs = 2Es (7) 
Thus the total energy used in pulhng a bar of unit cross section apart in 
such a way as to form two unit plane surfaces, is equal to twice the 
total surface energy. It is also equal to the force of surface cohesion 
{¥sc) integrated through the distance necessary to pull the two surfaces 
completely apart, or 
r»co 
Erp — I F^c ds 
Jso 
Energy of cohesion. — The cohesional energy of a solid or a liquid may 
be defined as the energy which would be liberated in the formation of 
the solid or liquid from its individual molecules, the molecules in the 
initial state being placed so far apart that they do not attract each 
other appreciably. This is equal to the latent heat of vaporization plus 
the heat absorbed in the expansion of the vapor until it becomes very 
dilute, minus the external work of vaporization, or it is equal to the 
internal latent heat of vaporization when the hquid is vaporized at 
a low pressure. 
