118 Mr. S. T. Preston on the Application of the 



presented itself to him after reading my two former papers on 

 this subject. As any theory that makes a pretention to truth 

 ought to court every criticism, I am glad to notice here the 

 difficulty alluded to, at the same time availing myself of the 

 opportunity to touch upon certain other points that would ap- 

 pear to want a little further elucidation. 



2. The point in question is, that, since gravity is proportional 

 to mass, it is admittedly necessary to assume that the total vo- 

 lume of free space in a substance must be great compared with 

 the total volume of matter contained in the molecules of the sub- 

 stance (in order that the medium producing gravity may be able 

 to penetrate the substance and act upon the molecules in the in- 

 terior). Mr. Croll finds a difficulty in reconciling this assump- 

 tion with some deductions regarding molecules by Sir William 

 Thomson, in a paper published in ' Nature,' vol. i. (p. 551). 

 Now I think it may be shown clearly here (and that this will 

 also be apparent to Mr. Croll on referring more minutely to 

 the wording of the above paper) that this paper was not in- 

 tended strictly to give molecular dimensions, but rather mole- 

 cular distances (from centre to centre), or number of molecules 

 in unit of volume. It is true that an estimate of molecular 

 dimensions is given on the special assumption that the radius 

 of a gaseous molecule is equal to " half the average shortest 

 distance reached in a vast number of collisions." Whether 

 this is the actual radius, therefore, depends evidently on 

 whether the two molecules come into contact at collision or not. 

 This might not be ; and if not, the radius might be smaller. 

 Thus it is at least conceivable that a layer of a medium may 

 exist between two vibrating approximated molecules, much as 

 a drop of water floats on a film of air. I do not wish to insist 

 upon this comparison ; but no one will, I think, consider that 

 it is necessary that molecules should come into contact ; and if 

 not, it is impossible to measure their dimensions, but only 

 their sphere of activity. This, therefore, would remove the 

 difficulty ; but I do not wish to hold necessarily to this expla- 

 nation, as it appears to me that there are some grounds for 

 supposing that molecules do come into contact. 



3. The explanation I have to bring forward is of another 

 character. The interstices are in the molecules themselves. 

 This explanation was also, I believe, suggested by Le Sage 

 himself. The old notion of a molecule being a hard spherical 

 mass certainly appears rather crude. In view of the numbers 

 of different capacities for vibration possessed by a molecule, 

 as proved by the spectroscope, it appears a necessary deduc- 

 tion that a molecule must be of a complex structure. Inter- 

 stices would make it complex. In ordinary architecture we 



