62 Mr. W. J. M. Rankine on the Mechanical Theory of Heat. 



as to the constitution of matter, called the Hypothesis of Mole- 

 cular Vortices. Its chief suppositions are the following : — 



1. That each atom of matter consists of a nucleus or central 

 physical point enveloped by an elastic atmosphere, which is re- 

 tained round it by attraction ; so that the elasticity of bodies is 

 made uj) 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 portions 

 of atmosphere condensed round them, and resisting not only 

 change of volume y but change oi figure also. 



2. That the changes of elasticity produced by heat arise from 

 the centrifugal force of revolutions or oscillations amongst 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. 



3. That the medium which transmits light and radiant heat 

 consists of the nuclei of the atoms vibrating independently, or 

 almost independently, of their atmospheres; absorptionheing the 

 transference of motion from the nuclei to the atmospheres, and 

 radiation 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 en- 

 tertained by others. 



The elasticity of the atomic atmosphere is supposed to vary 

 according to the law of Marriotte. 



The principal results arrived at in this introduction are the 

 following : — 



I. The specific gravities of the atomic atmospheres of all sub- 

 stances in the state of perfect gas are inversely proportional to 

 the coefficients of elasticity of those atmospheres. 



II. Quantity of heat is the vis viva of the motions of the par- 

 ticles of the atomic atnaospheres, whether rotations or rectilinear 

 oscillations. 



III. Temperature is proportional to 



(the velocity of the rotations)^ 



the coefficient of elasticity of the atmosphere ' 



the constant added being the same for all substances in nature. . 



IV. The maximum pressure of vapour in contact with its liquid 

 ia given by the following formula — 



where P is the pressure, t the temperature measured from a 

 point 274*^*6 Centigrade below the freezing-point of water, and 



