Relationship between Molecular Cohesion 495 



only the attraction of the upper layer and the distance as that 

 through which this layer would have been moved. It seems 

 to me that, for this reasoning to be correct, we must assume 

 the layers at least as thick as the radius of action of each layer 

 of molecules. If these layers are of molecular diameters, it 

 would seem that the cohesive force does not extend farther 

 than a molecular diameter. This is not apparently con- 

 sistent with the assumption elsewhere made to explain the 

 transitional layer between vapor and liquid. 



Plateau also followed Laplace. Sutherland, in his many 

 papers on molecular cohesion, assumes that the cohesional 

 attraction is inversely as the fourth power of the distance, but 

 he does not, so far as I can find, discuss the question of the 

 penetration of matter by cohesional attraction. But it is 

 impossible for cohesion to vary inversely as the fourth power, 

 if cohesion penetrates matter. 



Kleeman 1 supposes that the attraction must be in- 

 versely as the 5th or some higher power of the distance. He 

 has pointed out that cohesion cannot possibly be assumed to 

 vary like gravitation inversely with the square of the distance, 

 because the cohesional attraction is so much greater than 

 gravitation. But Mills weakened the force of this objection, 

 which he had overlooked in his early papers, by making the 

 additional postulate that cohesion does not penetrate matter. 

 For if it be assumed that the cohesion does not penetrate 

 matter then only the layer of superficial molecules of two 

 masses would attract each other; and the number of these is 

 so small, compared to the whole number of molecules in the 

 mass, that the cohesional attraction would be less perceptible 

 at sensible distances than gravitational attraction. 



I have been unable, then, to find any evidence for the 

 assumption so generally made that cohesive attraction pene- 

 trates matter like gravitation. There is no direct evidence, 

 therefore, so far as I can find, against the inference necessitated 



1 Kleeman: "An Investigation of the Determination of the Law of Chemical 

 Attraction between Atoms from Physical Data," Phil. Mag., [6] 21, 83 (1911). 



