468 M. Verdet's Historic Notice of 



gotten and neglected by all the world — the theory of the consti- 

 tution of gases to which I have referred towards the close of our 

 first meeting. His contemporaries saw probably in this theory 

 only a portion of the debris of ancient Cartesian hypotheses ; and 

 until quite recently nobody would suppose that in them would 

 be found the germ of a new science. 



In 1780, a little more than forty years after the publication 

 of the ' Hydrodynamics/ Lavoisier and Laplace, discussing in 

 their memoir on heat the two hypotheses which could be enter- 

 tained regarding the nature of this agent, express themselves in 

 the following manner : — 



" Other physicists think that heat is only the result of the 

 insensible vibrations of matter. ... In the system which we 

 are examining heat is the living force which results from the 

 insensible motions of a body, it is the sum of the products of the 

 mass of each molecule by the square of its velocity. . . . We 

 will not decide between the two preceding hypotheses; several 

 phenomena appear favourable to the latter, such, for example, as 

 the heat produced by the friction of two solid bodies; but 

 there are others which are more simply explained by the first 

 hypothesis; perhaps both of them occur at the same time." 



But after this assertion, so clear and so precise, in no part of 

 the memoir do we meet the idea of comparing the living force of 

 heat with the ordinary living force which is sensible in the 

 motion of a body's centre of gravity or in the motion of rota- 

 tion. Never did Lavoisier and Laplace compare heat except 

 with itself; and hence it mattered little for the fertility of their 

 reasonings, whether they considered heat as an indestructible 

 body, or as a quantity of living force. 



More than this, in a subsequent part of their work they 

 regard as evident a proposition which is directly opposed to the 

 principle of the conversion of heat into work. All variations of 

 heat, they say, whether real or apparent, experienced by a system 

 of bodies, in changing their state, are produced in the inverse order 

 when the system returns to its first condition. If they had added 

 that this equality occurs only when the changes of condition are 

 accompanied by no exterior work, the mechanical theory of heat 

 would have been founded; but without this complement the 

 assertion of Lavoisier and Laplace is an error, disproved every 

 day by the action of a steam-engine or an electro-magnetic ma- 

 chine. 



No one knows the extent to which the views of Lavoisier on 

 this subject might have been modified had he lived. It can 

 only be presumed, from the reading of his treatise on chemistry, 

 that in 1789 he had not entirely abandoned the theory which 

 refers heat to molecular motions. It is true that, yielding per- 



