CAPILLARY ATTRACTION. (APP. C) 71 



presented by the black spot which abruptly ends 

 the series of colours at places where the bubble is 

 thinnest before it breaks, make it quite certain that 

 the action of those forces becomes sensible at 

 distances not much less than a half wave length, 

 or 1/40000 of a centimetre. There is, indeed, much 

 and multifarious evidence that in ordinary solids 

 and liquids, not merely the distances of sensible 

 inter- molecular action, but the linear dimensions 

 of the molecules themselves, and the average 

 distance from centre to nearest centre, 1 are but 

 very moderately small in comparison with the 

 wave lengths of light. Some approach to a de- 

 finite estimate of the dimensions of molecules is 

 deducible from Clausius' theory of the average 

 spaces travelled without collision by molecules of 

 gases, and Maxwell's theory and experiments re- 

 garding the viscosity of gases. Having perfect 

 confidence in the substantial reality of the views 

 which these grand investigations have opened to 

 us, I find it scarcely possible to admit that there 

 can be as many as io 27 molecules in a cubic centi- 

 metre of liquid carbonic acid or of water. This 

 makes the average distance from centre to nearest 

 centre in the liquids exceed a thousand-millionth 

 of a centimetre ! 



We cannot, then, admit that the formulae which 



1 By "average distance from centre to nearest centre," I mean 

 the side of the cube in a cubic arrangement of a number of points 

 equal to the number of real molecules in any space. 



