6 Mr. W. J. M. Rankine on the Mechanical Action of Heat. 



their atmospheres ; — so that the absorption of light and of radiant 

 heat is the transference of motion from the nuclei to theii' atmo- 

 spheres, and the emission of hght and of radiant heat, the trans- 

 ference of motion from the atmospheres 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 compara- 

 tively slow velocity of propagation; yet in all probability the 

 minute \dbrations 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 pa-fect 

 solids, and with a rapidity corresponding to the extreme small- 

 ness 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 

 sether 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 in- 

 crease 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 recti- 

 linear 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 atmo- 

 sphere at what I have called the bounding sru'faces of the atoms, 

 and thus of augmenting the elasticity of the body. 



In this summary, I shall not enter into the details of mathe- 

 matical analysis, but shall state results only. The following, 



* The consequences of this supposition, in the theory of polarization and 

 double refraction, are pointed out in a pai)er read to the Royal Society of 

 Edinburgh ou the 2nd of December 1850, and published in the Philoso- 

 phical Magazine for June 1851. A modification of this third supposition, 

 according to which the motion constituting light is an oscillation of the 

 atomic nuclei round transverse axes, and is transmitted by means of a 

 species of polarity, forms the subject of a paper read to the British Asso- 

 ciation at Hull, and published in the Philosophical Magazine for Decem- 

 ber 185:1 



