238 Mr. E. H. Cook on the Existence of 



It is impossible not to be struck with the relation which is 

 here exhibited, especially when we remember how many in- 

 fluences are at work interfering with the perfect freedom of 

 motion which is necessary for this law to be rigorously true. 

 Numerous confirmations of this molecular theory occur when 

 we examine tables of the refractive indices of various bodies. 

 We extract the following passages from the article " Light" 

 in e Watt's Dictionary of Chemistry/ vol. iii. pp. 616-618 : — 



" Generally speaking, the refractive power of any one sub- 

 stance increases with its density. - " 



" The refracting power of liquids is diminished when they 

 are expanded by heat." 



Biot and Arago " found that at pressures not exceeding that 

 of the atmosphere, the quantity /ul 2 — 1, which is called the ab- 

 solute refractive power, is proportional to the density of the 

 gas." 



u Dulong has shown that the refractive power of a mixture 

 of gases is equal to the mean of those of the constituent gases 

 calculated for the pressure to which each gas is actually sub- 

 jected in the mixture." 



Compare the self-evident explanation offered of these results 

 on the molecular theory with the complicated and unsatisfactory 

 nature of that afforded by the ether theory, even after assuming 

 the existence and all-pervading properties of this substance. 



Again, in the complicated phenomena of interference and 

 polarization, how few are the assumptions which we have to 

 make ! No difference in elasticity of a contained medium, 

 owing to the different molecular groupings, but these diffe- 

 rent molecular groupings themselves all-sufficient to account 

 for the phenomena. 



Colour and chemical action are also found to be very much 

 more easily explained, when we consider the molecules of 

 bodies to vibrate instead of the molecules of the ether. 



Summing these conclusions, we have : — 



a. The molecular theory makes no departure in its assump- 

 tions from the analogies observed in nature. 



j3. The phenomena of refraction follow as a consequence of 

 this theory. 



7. The complicated phenomena of colour, double refrac- 

 tion, polarization, and interference are all explained without 

 making assumptions which have no analogies in observed 

 facts. 



6\ Independent phenomena, especially the increase of the 

 refractive powers of gases with the increase of their densities, 

 support this theory. 



e. The turning of the plane of polarization by the passage 



