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VII.—On the Mechanical Action of Heat, especially in Gases and Vapours. By 
Witii1aM Jonn Macquorn RANKINE, Civil Engineer, F.R.S.E., F.R.S.S.A., &c. 
(Read 4th February 1850.) 
INTRODUCTION. 
SUMMARY OF THE PRINCIPLES OF THE HYPOTHESIS OF MOLECULAR VORTICES, AND ITS APPLICA- 
TION TO THE THEORY OF TEMPERATURE, ELASTICITY, AND REAL SPECIFIC HEAT. 
The ensuing paper forms part of a series of researches respecting the conse- 
quences of an hypothesis called that of Molecular Vortices, the object of which is, 
to deduce the laws of elasticity, and of heat as connected with elasticity, by means 
of the principles of mechanics, from a physical supposition consistent and con- 
nected 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 experiments of M. 
REGNAULT on gases and vapours. 
The investigation which I have now to describe, relates to the mutual con- 
version 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 sum- 
mary of the principles of the hypothesis as is necessary to render the subsequent 
investigation intelligible. 
The fundamental suppositions are the following :— 
First,—That each atom of matter consists of a nucleus, or central physical 
point, enveloped by 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, A¢PI- 
__ nus, Mossorvi, and others. They have in general, however, conceived the atmo- 
sphere 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 
VOL. XX. PART I. 2k 

