486 Mr. E. H. Cook on the 



transmission of light through them, then it is a natural result 

 that the refractive powers of such bodies should be the same. 

 But if, on the contrary, we consider the aether which fills the 

 spaces between these molecules as the medium which trans- 

 mits the light- waves, then there is no simple connexion be- 

 tween the two. We have, if we wish to establish any con- 

 nexion whatever, to make two additional assumptions, which 

 it is easy to show are not justified by facts. These assump- 

 tions are, first, that the spaces between the molecules of these 

 bodies are of equal size; and, secondly, that the shape of the 

 spaces is the same in all bodies, or else the freedom of the aether 

 filling these spaces to vibrate would be different. It is evi- 

 dent that these assumptions are necessary; for if in one body 

 the spaces filled by the aether are larger than in another, then 

 we should have that the freedom of the aether to vibrate is 

 greater in one case than in another, and therefore that the 

 refractive powers are different. Now we know from experi- 

 ments upon isomeric and metameric bodies, that the specific 

 gravity of these bodies is not constant. It varies considerably 

 in some cases. For example, the specific gravity of allylic 

 alcohol is '8709 at 0° C, and that of acetone, isomeric with it, 

 is '792. But since the molecules are made up of the same 

 chemical elements united in the same proportions, these mole- 

 cules have the same density. The difference in density must 

 therefore arise from a difference in the mean distances apart 

 of these molecules; and thus the spaces enclosed by the mole- 

 cules must be of different size. Again, are the spaces of the 

 same shape ? The constitutional formulae of chemical com- 

 pounds are deduced from the way these compounds behave 

 when treated with chemical agents. Although no chemist 

 would be found to say that it was a matter of absolute truth that 

 the atoms were arranged in the molecule in the manner shown 

 by its constitutional formula, yet in very many organic com- 

 pounds the abundance of accumulated evidence is so vast that 

 it is a matter of great probability that such is the case. Thus 

 we gain an insight into the shape of the molecules imbedded 

 in the aether, and therefore also of the shape of the spaces 

 between these molecules. The constitutional formulae of iso- 

 meric and metameric bodies are, as is well known, very various. 

 In fact, isomeric bodies are distinguished in chemical formulae 

 by the difference in their constitutional formulae. There can be 

 no doubt, therefore, that the spaces between the molecules are 

 not of the same shape; and thus the assumption is incorrect. 

 The investigation of the first law of Gladstone and Dale thus 

 reveals insuperable difficulties in the assumption of the inter- 

 molecular existence of the aether. All the remaining laws are 



