277 



Thirdly, What are the figures of the orbits described by the par- 

 ticles of the atomic atmospheres in tlioir revolutions or oscillations ? 



The author introduces into the hypothesis of molecular vortices a 

 supposition peculiar to his own researches, for the purpose of con- 

 necting it with the undulatory hypothesis as to radiation. It is this : 

 TItat the vibration which, according to the undulatory hypothesis, 

 constitutes radiant light and heat, is a motion of the atomic nuclei 

 or centres, and is propagated by means of their mutual attractions 

 and repulsions. The absorption of light and of radiant heat, accord- 

 ing to this supposition, is the transference of motion from the nuclei 

 or centres to their atmospheres, and the emission of light and ra- 

 diant heat, the transference of motion from the atmospheres to the 

 nuclei or centres. The author enumerates several advantages which 

 he conceives that this hypothesis possesses over the common supposi- 

 tion of a luminiferous ether pervading the spaces between ponderable 

 particles. 



The present paper refers solely to the condition of bodies in the 

 state of gas or vapour. It is divided into two parts, the first of 

 which treats of the Statical Relations of Heat and Elasticity, or 

 their relations when both are invariable ; and the second, of their 

 Dynamical Relations, which take place when gaseous bodies expand 

 or contract, and involve the principles of the mutual conversion of 

 heat and expansive power, and those of the latent heat of expan- 

 sion and evaporation. 



The first section of the first part explains the general principles 

 of the hypothesis, of which a summary has just been given. 



The second section contains the mathematical investigation of the 

 general equation between the heat and the elasticity of a gas. 

 The total elasticity is divided into two parts, — the superficial 

 atomic elasticity, being the elasticity of the atomic atmospheres at the 

 bounding surfaces of the atoms, which is always expansive, and a 

 function of density and heat, and an elasticity arising from the 

 mutual forces exerted by separate atoms, which may be expansive 

 or contractive, and in the perfectly fluid state is a function of den- 

 sity only. 



The more substances are rarefied, that is to say, the more the 

 forces which interfere with the operation of the elasticity of the 

 atomic atmospheres are weakened, the more nearly do they approach 

 to a condition called that of perfect gas, in which the total elasticity 



