Plane-Polarized Light. 443 



facts which have thus been established, I now offer the following 

 suggestions. 



In a paper read to the Royal Society of Edinburgh, and pub- 

 lished in their Transactions, vol. xx. part 1, I proposed as a 

 foundation for the theory of heat, and of the elasticity of gases 

 and vapours, an hyjiothesis called that of molecular vortices; 

 and in a subsequent paper, already referred to, I deduced from 

 the same hjrpothesis some principles relative to the elasticity of 

 solids. I shall now show that FresneFs conjecture as to the 

 dii'ection of vibration in plane-polarized hght is a natui'al con- 

 sequence of that hyjjothesis. 



The fundamental suppositions of the hypothesis of molecular 

 vortices are the following : — 



First, That each atom of matter consists of a nucleus or cen- 

 tral physical point enveloped by an elastic atmosphere, which is 

 retained round it by attraction ; so that the elasticity of bodies 

 is made up of two parts ; one arising from the diffused portion 

 of the atmospheres, and resisting change of volume only ; the 

 other arising from the mutual actions of the nuclei, and of the 

 ])ortions of atmosphere condensed round them, and resisting not 

 only change of volume but also change of figui'e. 



Secondhj. That the changes of elasticity due to heat arise from 

 the centrifugal force of revolutions or oscillations among the 

 particles of the atomic atmospheres, diffusing them to a greater 

 distance from their nuclei, and thus increasing the elasticity 

 which resists change of volume only, at the expense of that 

 which resists change of figure also. 



Thirdly. That the medium which transmits light and radiant 

 heat consists of the nuclei of the atoms, vibrating independently, 

 or almost independently, of their atmospheres; absorption being 

 the transfei'ence of motion from the nuclei to the atmospheres, 

 and emission its transference from the atmospheres to the nuclei. 



This last supposition is peculiar to my own researches, the 

 first two having more or less resemblance to ideas previously 

 entertained by others. 



If an indefinitely extended vibrating medium, equally elastic 

 in all directions, consist of a system of atomic nuclei, tending to 

 preserve a certain configuration in consequence of their mutual 

 attractions and repulsions, it is well known that such a medium 

 is capaVjle of transmittmg two sorts of vibrations only, longitudinal 

 and transverse ; the latter alone being supposed to be concei'ned 

 in the phsenomena of light. It is also well known, that the 

 square of the velocity of propagation of transverse vibrations is 

 directly proportional to a quantity called the transverse elasticity 

 of the medium, arising from the mutual actions of the nuclei, 

 and inversely proportional to its density, that is, to the sum of 

 the masses of all the nuclei contained in unity of volume. To 

 2 H 2 



