148 MR W. J. M. RANKINE ON THE 
centres of the body balance each other; and the elasticity of the body is made up 
of two parts: First, the elasticity of the atomic atmospheres at the imaginary 
boundaries of the atoms, which I shall call the superficial-atomic elasticity ; and, 
secondly, the force resulting from the mutual actions of distinct atoms. If the 
atmospheres are so much condensed round their nuclei or centres, that the super- 
ficial-atomic elasticity is insensible, and that the resultants of the mutual actions 
of all parts of the distinct atoms are forces acting along the lines joining the 
nuclei or centres, then the body is a perfect solid, having a tendency to preserve 
not only a certain bulk, but a certain figure; and the elasticity of figure, or rigid- 
ity, bears certain definite relations to the elasticity of volume. 
If the atmospheres are less condensed about their centres, so that the mutual 
actions of distinct atoms are not reducible to a system of forces acting along the 
lines joining the atomic centres, but produce merely a cohesive force sufficient to 
balance the superficial-atomic elasticity, then the condition is that of a perfect 
liquid; and the intermediate conditions between this and perfect solidity consti- 
tute the gelatinous, plastic, and viscous states. 
When the mutual actions of distinct atoms are very small as compared with 
the superficial-atomic elasticity, the condition is that of gas or vapour ; and when 
the substance is so far rarefied that the influence of the atomic nuclei or centres 
in modifying the superficial elasticity of their atmospheres is insensible, it is then 
in the state of perfect gas. 
So far as our experimental knowledge goes, the elasticity of a perfect gas at 
a given temperature varies simply in proportion to its density. I have therefore 
assumed this to be the law of the elasticity of the atomic atmospheres, ascribing 
a specific coefficient of elasticity to each substance. 
The second supposition, being that from which the hypothesis of molecular 
vortices derives its name, is the following :—T7hat the elasticity due to heat arises 
Jrom the centrifugal force of revolutions or oscillations among the particles of the 
atomic atmospheres ; so that quantity of heat is the vis viva of those revolutions or 
oscillations. 
This supposition appears to have been first definitely stated by Sir Humpury 
Davy. It has since been supported by Mr Joutz, whose valuable experiments to 
establish the convertibility of heat and mechanical power are well known. So 
far as I am aware, however, its consequences have not hitherto been mathema- 
tically developed. 
To connect this hypothesis with the undulatory theory of radiation, I have 
introduced a third supposition:—That the medium which transmits light and 
radiant heat consists of the nuclei of the atoms, vibrating independently, or almost 
independently, of their atmospheres ;—so that the absorption of light and of radiant 
heat, is the transference of motion from the nuclei to their atmospheres, and the 

