



MECHANICAL ACTION OF HEAT. 149 
emission of light and of radiant heat, the transference of motion from the atmo- 
spheres to their nuclei. 
Although in all undulations of sensible length and amplitude, such as those 
of sound, the nuclei must carry their atmospheres along with them, and vibrating 
thus loaded, produce a comparatively slow velocity of propagation; yet in all 
probability the minute vibrations of light and radiant heat may be performed by 
the atomic nuclei in transparent and diathermanous bodies, without moving the 
atmospheres more than by that amount which constitutes absorption; and those 
vibrations will therefore be transmitted according to the laws of the elasticity of 
perfect solids, and with a rapidity corresponding to the extreme smallness of the 
masses set in motion, as compared with the mutual forces exerted by them. 
This supposition is peculiar to my own view of the hypothesis, and is, in fact, the 
converse of the idea hitherto adopted, of an ether surrounding ponderable particles. 
The second and third suppositions involve the assumption, that motion can 
be communicated between the nuclei and their atmospheres, and between the 
different parts of the atmospheres; so that there is a tendency to produce some 
permanent condition of motion, which constitutes equilibrium of heat. It is now 
to be considered what kind of motion is capable of producing increase of elasticity, 
and what are the conditions of permanency of that motion., 
It is obvious, that the parts of the atomic atmospheres may have motions of 
alternate expansion and contraction, or of rectilinear oscillation about a position 
of equilibrium, without affecting the superficial atomic elasticity, except by small 
periodical changes. Should they have motions, however, of revolution about 
centres, so as to form a group of vortices, the centrifugal force will have the effect 
of increasing the density of the atmosphere at what I have called the bounding 
surfaces of the atoms, and thus of augmenting the elasticity of the body. 
In this summary, I shall not enter into the details of mathematical analysis, 
but shall state results only. The following, then, are the conditions which 
must be fulfilled, in order that a group of vortices, of small size as compared with 
the bulk of an atom, and of various diameters, may permanently coexist, whether 
side by side, or end to end, in the atomic atmospheres of one substance, or of 
various substances mixed. 
First, The mean elasticity must vary continuously ; which involves the condi- 
_ tion, that at the surface of contact of two vortices of different substances, side by 
side, or end to end, the respective densities at each point of contact must be 
inversely proportional to the coefficients of elasticity. Hence the specific gravities 
of the atmospheric parts of all substances, under precisely similar circumstances as to 
heat and molecular forces (a condition realised in perfect gases at the same pres- 
sure and temperature), a7¢ inversely proportional to the coefficients of atmospheric 
elasticity. Therefore let » represent the mass of the atmosphere of one atom of 
