Relationship between Molecular Cohesion 485 



sometimes the same factors as the value a/V 2 of van^der 

 Waals' equation. I have not, however, attempted to estab- 

 lish any closer connection between them. 1 



We conclude, therefore, that while cohesion and re- 

 fractivity are both dependent on a common factor, namely 

 the valence electrons, and possibly upon the molecular weight, 

 the connection between them is not direct, but indirect; 

 and while cohesion and refraction, or dispersion, often parallel 

 each other, they, at other times, diverge considerably since 

 other factors enter into refraction. 



It is not without interest to recall as an example of the 

 perspicacity of genius, that I^aplace 2 long ago foretold a con- 

 nection between these properties. Writing in 1805 of the 

 formula of capillarity which, as will be remembered, con- 

 tained two terms, -one K, representing molecular cohesion, 

 or van der Waals' expression a/V; the other, H, the capillary 

 constant, Laplace says (p. 351): "I saw that this action 

 (pressure) is smaller or larger than if the surface is plane; 

 smaller, if the surface is concave; larger, if it is convex. Its 

 analytical expression is composed of two terms: the first 

 (K), much larger than the second, expresses the action of the 

 mass terminated by a plane surface; and I think from this 

 term depends the suspension of mercury in a barometer tube 

 at a height two to three times greater than that due to 

 atmospheric pressure, the refractive powers of diaphanous 

 bodies, the cohesion, and in general, chemical affinity; the 

 second term expresses the part of the action due to the spheri- 

 city of the surface." And again (p. 362): "The function, K, 

 is analogous to that I have designated by the same letter in 

 the refraction of light." 



But of even greater interest and more fundamental 

 importance than the relation between the optical and the 

 cohesive properties, which is now understandable since both 



1 See also Natanson: Bull, de 1'Acad. des Sci. de Cracovie, 1907, April 

 p. 316 for the relation of refraction and valence. 



2 Laplace: Sur 1'action capillaire. Oeuvres. Supp. Liv. X, Trait de 

 Mecanique Celeste, p. 351. 



