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



Vortices, the object of which is, to deduce the laws of elasticity, 

 and of heat as connected with elasticity, by means of the prin- 

 ciples of mechanics, from a physical supposition consistent and 

 connected with the theory which deduces the laws of radiant 

 light and heat from the hypothesis of undulations. Those 

 researches were commenced in 1842, and after having been laid 

 aside for nearly seven years, from the want of experimental data, 

 were resumed in consequence of the appearance of the experi- 

 ments of M. Regnault on gases and vapours. 



The investigation which I have now to describe, relates to the 

 mutual conversion of heat and mechanical power by means of 

 the expansion and contraction of gases and vapours*. 



In the introduction which I here prefix to it, I purpose to 

 give such a summary of the principles of the hypothesis as is 

 necessary to render the subsequent investigation intelligible. 



The fundamental suppositions are the following : — 



Fiist, — That each atom of matter consists of a nucleus, or cen- 

 tral physical point, enveloped brj an elastic atmosphere, which is 

 retained in its position by forces attractive towards the nucleus or 

 centre. 



Suppositions similar to this have been brought forward by 

 Franklin, iEpinus, Mossotti, and others. They have in general, 

 however, conceived the atmosphere of each nucleus to be of 

 variable mass. I have treated it, on the contrary, as an essential 

 part of the atom. I have left the question indeterminate, whether 

 the nucleus is a small body of a character distinct from that of 

 the atmosphere, or merely a portion of the atmosphere in a highly 

 condensed state, owing to the mutual attraction of its parts. 



According to this first supposition, the boundary between two 

 contiguous atoms of a body is an imaginary surface at which the 

 attractions of all the atomic 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 con- 

 densed round their nuclei or centres that the superficial-atomic 

 elasticity is insensible, and that the resultants of the mutual 



the original paper ^re incorporated some corrected numerical results for- 

 merly contained in a supplementary pajier read on the 2nd of December 

 1850, which were deduced from Mr. Joule's latest experiments on the me- 

 chanical equivalent of heat. The foot-notes have all been added since the 

 original paper was read.] 



• The statical relations between the density, heat, temperature, and pres- 

 sure of gaseou* bodies, as deduced from this hypothesis, form the subject 

 of a paper "On the Centrifugal Theory of Elasticity," published in the 

 Philosophical Magazine for December 1851. 



