122 DIRECT PROPEETIES OF MOLECULES 337 



spends sufficiently accurately to the size of the gaseous 

 molecules which we have obtained from the kinetic theory, 

 viz. 0'2 millionth of a millimetre for the diameter of a 

 molecule. 



The experiments by which it has been attempted to de- 

 termine the size of the spheres of action of the molecular 

 forces in liquids, or the distance to which capillary forces 

 sensibly extend, also afford help towards the answering of 

 our question. For this purpose Plateau 1 measured the 

 smallest thickness at which films of a soap solution contain- 

 ing gtycerine could still hold together ; he took a half of 

 this thickness as the range of the molecular forces, and thus 

 obtained the value 0*00005 mm. This value of the range 

 of the molecular forces was also found by Quincke 2 

 by measuring the thickness of thin layers on a wall which 

 exerted capillary force on a liquid through the thin layer. 

 Sohncke 3 proceeded by Plateau's method, and measured 

 the smallest thickness of a layer of oil which spread itself 

 over the surface of a liquid ; he, like Plateau and Quincke, 

 also found 0-00005 mm. for the semi-thickness. But 

 shortly after Kontgen 4 showed that it is possible to obtain 

 much thinner layers of oil ; he observed layers of only 0'56 

 millionth of a millimetre which held together, so that the 

 range of the capillary forces in Plateau's sense would be 

 only 0'3 millionth of a millimetre, and would consequently 

 agree almost exactly with the diameter of the molecular 

 sphere of a gaseous particle. Drude 5 found that, when a 

 film made of Plateau's soap solution has become so thin 

 as to show no colour, but blackness only, it is 17 millionths 

 of a millimetre thick at this point, so that the range of the 

 capillary forces would be half of this, or 8J millionths of a 

 millimetre/' E e i n o 1 d and K ii c k e r 7 had, by measurements 



1 -Mem. de Brux. 1861, xxxiii, p. 44 ; 1847, xvi. p. 35. 



2 Pogg. Ann. 1869, cxxxvii. p. 402. 



3 Milncli. Sitzungsber. 1890, xx. p. 93 ; Wied. Ann. 1890, xl. p. 345. 



4 Wied. Ann. 1890, xli. p. 321. 

 ' 5 Ibid. 1891, xliii. p. 158. 



Johonnott estimates 6 x 10~ 6 mm. (Phil. Mag. 1899 [5] xlvii. p. 501). 

 7 Proc. Roy. Soc. 1877, xxvi. p. 334 ; Wied. Ann. 1891, xliv. p. 778. 



Z 



