CHEMISTRY: W. D. HARKINS 
565 
Internal pressure or cohesion.— The internal pressure of a liquid or a 
solid has been defined as the rate of transfer of momentum across 
a unit plane area inside the liquid or solid; and the average force of 
attraction across this unit area, which is numerically equal to the 
internal pressure, is the cohesional force, or the cohesion. 
While the work and total energy of adhesion and of surface cohesion, 
and the energy of cohesion, may all be obtained from experimental 
results by the use of simple and exact thermodynamic equations, this 
is not true of the internal pressure or cohesion. As a matter of fact, 
there is at present no known means of calculating the cohesion, but 
there are many methods, which do not agree among themselves, of 
calculating from inexact equations, values which for various liquids are 
supposed, when arranged in order of magnitude, to lie in the same 
order in general as the cohesions themselves. In fact, the cohesion is 
often defined as equal to a/v,'^ the pressure correction term in van der 
Waals equation. However, since this equation is far from exact in its 
application to liquids, it is obvious that the cohesion calculated cannot 
represent at all accurately the internal pressure. 
Molecular attraction. — All of the phenomena thus far considered in 
this paper may be considered as due to the attraction between the 
molecules in a liquid or a solid. It is customary to consider the mole- 
cules as spheres or as points, with the attractive forces dependent upon 
the distance between the molecules alone, when they are all alike. It 
has been shov/n by Harkins, Brown, and Davis,-^ by a measurement of 
the amounts of energy involved, and by Langmuir^ by a less direct 
method, that the forces around different parts of a molecule may be 
very different in magnitude. Thus in the case of organic compounds 
the forces are very much higher around any groups containing oxygen, 
nitrogen, triple, or double bonds, than they are around the hydrocarbon 
chains. The investigations of Harkins, Grafton and Ewing (these Pro- 
ceedings, 5, 1919,571) show that if organic substances are arranged ac- 
cording to the magnitude of their adhesional surface work toward mer- 
cury, they are not so arranged with respect to water. In this respect the 
adhesional forces seem to have something of the specific nature which 
indicates chemical action, and it is well known that the recent work on 
crystal structure demonstrates that such crystals as those of diamond 
and of graphite are held together by primary valence bonds. Lang- 
muir^ considers all cohesional and adhesional forces as chemical, while 
van Laar^ has recently published the results of an extensive series of 
calculations which show that the square root of van der Waals' con- 
