Relationship between Molecular Cohesion 489 



cohesion. I may state briefly some of the reasons which ap- 

 pear to lead to such a conclusion. 



In the first place we have the surprising fact that the 

 field of cohesion of a molecule is apparently delimited by 

 the surrounding molecules. The evidence for this, while 

 perhaps not conclusive, is both direct and indirect. The 

 reason for the shortness of the radius of action of cohesion is 

 one of the most interesting questions of molecular physics. 

 It is of interest to see how this question came to be generally 

 considered closed and settled in favor of the view, now gener- 

 ally accepted, that cohesional attraction diminishes with the 

 distance at a rate far greater than gravitational attraction. 

 It is chiefly due to Laplace. 



Laplace, 1 in his beautiful memoir on capillarity, first 

 raised the question whether the short radius of attraction 

 of the cohesive forces was due to the fact that matter shut 

 off the attraction, or was due to the attraction diminishing 

 with the distance at a rate far more rapid than gravitation. 



He says, when discussing Hawksbee's well-known ex- 

 periments proving that the height to which water rises in a 

 glass tube is independent of the thickness of the wall of the 

 tube: "Hawksbee a observe que dans les tubes de verre, 

 ou tres minces ou tres epais, 1'eau s'elevait a la meme hauteur 

 toutes les fois que les diametres interieurs etaient les memes. 

 Les couches cylindriques du verre qui sont a une distance 

 sensible de la surface interieure ne contribuent done point a 

 1' ascension de 1'eau, quoique dans chacune d'elles, prise 

 separement, ce fluide doive s'elever au-dessus du niveau. 

 Ce n'est point 1'interposition des couches qu'elles embrassent 

 qui arrete leur action sur 1'eau, car il est naturel de penser 

 que les attractions capillaires se transmettent a tracers les corps, 

 ainsi que la pesanteur; cette action ne disparait done qu'a raison 

 de la distance du fluide a ces couches, d'ou il suit que V attraction 

 du uerre sur Veau n'est sensible qu'a des distances insensibles ." 

 I have italicized the end of Laplace's statement to bring out 



1 Laplace: "Sur 1'action capillaire. Oeuvres. Supp. au Livre X," Traite de 

 Mecanique Celeste, p. 351; see also p. 487. 



