496 Albert P. Mathews 



by the square or fourth power law of attraction, that cohesion 

 does not penetrate matter. 



There is, on the other hand, some evidence of a direct kind 

 that cohesional attraction extends only as far as the nearest 

 molecules. This evidence is the length of the radius of action 

 as determined by direct measurement, and Einstein's proof 

 that the radius of action varies with the distance apart of the 

 molecular centers. 



Laplace believed that the radius of action, although 

 short, nevertheless extended many molecular diameters and 

 this opinion prevailed until recently, but as means of measure- 

 ment have improved the radius has shrunk. Quincke gave an 

 estimate of about 6 X io~ 6 cm, but the most recent deter- 

 minations of Johannot, 1 and Chamberlain 2 show it to be 

 about 1.6-2 X io~ 7 cm in a soap film and in the case of glass. 

 The diameter of a molecule of trioleate of glycerine, according 

 to Perrin, 3 is i.i X io~ 7 cm. The radius of action is certainly 

 not more than two molecular diameters and indeed is hardly 

 more than one. The average distance between the centers 

 of two molecules of ether in the liquid state at 20 is about 

 5.5 X io- 8 cm. 



But not only has direct measurement shown the radius 

 of action to be one or two molecular diameters, but com- 

 putations of it by Kleeman make it very close to this. For 

 example, in ether Kleeman 4 computed the radius to be about 

 3.4 X io~ 8 cm which is about the distance when the mole- 

 cules are in contact. Van der Waals supposed it, indeed, to 

 be only as long as this. Recently Einstein 5 has made a very 

 interesting computation starting from the law of Eotvos, by 

 which he shows, from thermodynamic reasoning, that -the 



1 Johannot: Phil. Mag., [5] 47, 501 (1899). 



2 Chamberlain: Phys. Rev., 31, 170 (1910). 



3 Devaux: Journal de Physique, [5] 2, 699 (1912). 



4 Kleeman: "On the Radius of the Sphere of Action," Phil. Mag., [6] 19, 

 840 (1910). 



8 Einstein: "Bemerkung zu dem Gesetz von Eotvos," Annalen der Physik., 

 [4] 34, 165 (19-11). 



