1885.] on Liquid Films. 24'J 



film is formed by lowering tlie whole into the liquid with which the 

 lower part of the vessel is flooded. The light reflected from the 

 film is passed through a lens, and au image formed upon a screen. 

 When the bands of colour are seen descending from the upper part of 

 the film a current from 50 Grove's cells is passed through it. If the 

 current flows downwards the bands of colour move more quickly than 

 before ; if it flows upwards their motion is checked and they begin 

 to ascend. The cause of this curious fact is still unknown. It may 

 either be analogous to the j^benomenon known as the "migration of 

 the ions," or it may be a secondary effect" due to a change in the sur- 

 face tension. 



The general relation of the results attained in these investigations 

 as to the question of the size of molecules is interesting. Sir William 

 Thomson has expressed the opinion that 2 X 10~^ mm. and 

 0.01 X 10~'^mm. are superior and inferior limits respectively to 

 the diameter of a molecule. Van der Waals has been led, from con- 

 siderations founded on the theory of gases, to give 0* 28 X lO""*^ mm. 

 as an approximate value of the diameters of the molecules of the 

 gases of which the atmosphere is composed. The number of mole- 

 cules which could be plnced side by side within the thickness of the 

 thinnest soap film would, according to these various estimates, be 

 4, 26, and 720 respectively. The smallness of the first of these 

 numbers, especially when it is remembered that the liquid used on 

 some occasions w^as of a highly complex character, containing water, 

 glycerine and soap, points to the conclusion that the diameter of a 

 molecule is considerably less than 2 x 10 ~^ mm. 



[A. W. E.] 



