299 
of Edinburgh, Session 1873-74. 
further suppositions that the whirling motion is diffused in the 
form of atmospheres round nuclei, which may be either bodies of 
a special kind or centres of condensation and attraction in the 
atmospheres ; and that radiance, whether of heat or light, consists 
in the transmission of a vibratory motion of the nuclei, by means 
of forces which they exert on each other. 
The quantity of heat in a body is the energy of its molecular 
vortices ; the absolute temperature of the body is the same energy 
divided by a specific co- efficient for each particular substance. A 
perfect gas is a substance in which the elastic pressure is sensibly 
that which varies with the centrifugal force of the vortices only ; 
and the intensity of the pressure, according to the known principles 
of mechanics, must be proportioned directly to the energy of 
the vortices, and inversely to the space that they occupy. In 
substances not perfectly gaseous , the elasticity is modified by 
attractive or cohesive forces. When the deviation from the per- 
fectly gaseous state is small, the effects of such forces may be 
approximately represented by series, in terms of the reciprocal of 
the absolute temperature. Eankine had previously published an 
example of the use of such series, in a paper on the Elasticity 
of Vapours (Edin. Phil. Journal, July 1849), and he also applied 
them with success to the elasticity of carbonic acid and some other 
gases (Phil. Mag. 1851). Sensible heat is the energy employed in 
varying the velocity of the whirling particles ; latent heat the work 
done in varying the dimensions of their orbits, when the volumes 
and figures of the spaces in which they whirl are changed. The 
force which keeps any particle in its orbit is equal and opposite 
to the centrifugal force of that particle; therefore the work done 
in varying the orbits of the particles is proportionate to their 
centrifugal forces, therefore to the energy of the vortices, there- 
fore to the absolute temperature. And to compute that quantity 
of work, or latent heat, when a body undergoes a given variation 
of dimensions, the absolute temperature is to be multiplied by the 
corresponding variation of a certain function of the dimensions 
and elasticity of the body. This function is computed by taking 
the rate of variation with temperature, of the external work done 
during the kind of change of dimensions under consideration. 
Such is an outline of the method by which Rankine deduces 
VOL. VIII. 2 Q 
