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periments of M. Regnault. The author has thus been enabled to 

 resume his investigations, and has obtained formulae, agreeing with 

 experiment, and applicable to practice, for the expansion and elas- 

 ticity of gases, — the elasticity of vapours in contact with their 

 liquids, — the specific heat of gases, — tho heat produced by their 

 compression, — the latent and total heat of evaporation, — the expan- 

 sive action of vapours, — the power of the steam-engine, — and the 

 mechanical value of heat in general. 



One of the most useful in practice of those formulae, — that for cal- 

 culating the elasticity of steam and other vapours in contact with 

 their liquids, — was published separately in the Edinburgh New 

 Philosophical Journal for July 1849, with tables and a diagram, 

 shewing its agreement with experiment, but without any account of 

 the reasoning from which it is deduced. 



The theory of radiant heat, like that of light, having been re- 

 duced to a branch of mechanics by means of the hypothesis of undu- 

 lations, it is the object of the hypothesis of Molecular Voi'tices to 

 reduce the theory of stationary heat also to a branch of mechanics, 

 and thus to make a further step towards the fulfilment of the wish 

 of Newton, — " Utinam cetera naturae phenomena ex prin- 



CIPIIS MECHANICIS DERIVARE LICERET." 



The hypothesis of molecular vortices is defined to be that which 

 assumes, that each atom of matter consists of a nucleus or central 

 point, enveloped hy an elastic atmosphere, which is retained in its 

 position hy attractive forces, and that the elasticity due to heat 

 arises from the centrifugal force of those atmospheres, revolving or 

 oscillating about their nuclei or central points. According to this 

 hypothesis, quantity of heat is the vis viva of the molecular revolu- 

 tions or oscillations. 



The author, for the pi-esent, leaves indeterminate the following 

 questions, as he has not as yet found it necessary to make any definite 

 supposition respecting them. 



First, Whether the elastic molecular atmosphei'es are continuous, 

 or consist of discrete particles \ This includes the question, Whether 

 expansive elasticity is wholly the result of the mutual repulsions of 

 particles, or is, to a certain extent, a primary quality of matter ? 



Secondly, Whether, at the centre of each atom, there is a real 

 nucleus or extremely small central body, or a mere centre of con- 

 densation and force? 



