68 Proceedings of the Royal Society 
bubbles. But the appearances 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 3-5-0 -gr 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,* are but very moderately 
small in comparison with the wave lengths of light. Some 
approach to a definite 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 regarding 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 10 27 molecules in 
a cubic centimetre 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 I have given 
above are applicable to express the law of equilibrium between the 
moisture retained by vegetable substances, such as cotton cloth or 
oatmeal, or wheat-flour biscuits, at temperatures far above the 
dew point of the surrounding atmosphere. But although the 
energy of the attraction of some of these substances for vapour 
of water (when, for example, oatmeal, previously dried at a high 
temperature, has been used, as in the original experiment of Sir J. 
Leslie, to produce the freezing of water under the receiver of an air- 
pump), is so great that it might almost claim recognition from 
chemists as due to a “ chemical affinity,” and resulting in a “ chemi- 
cal combination,” I believe that the absorption of vapour into 
fibrous and cellular organic structures is a property of matter 
continuous with the absorption of vapour into a capillary tube 
demonstrated above. 
* 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. 
