THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



SUPPLEMENT to VOL. V. FOURTH SERIES. 



f 



LXXV. Researches upon the Specific Heat of Elastic Fluids*, 

 By M. V. Regnault. 



FOR more than twelve years I have been occupied in col- 

 lecting the data necessary for the solution of the following 

 problem : — 



What is theoretically the motive power which may be ob- 

 tained from a given quantity of heat by applying it to the deve- 

 lopment and dilatation of various elastic fluids under the various 

 conditions which are practically realizable ? 



The complete solution of this problem should furnish not 

 merely the true theory of the steam-engines used at the present 

 day, but likewise that of machines in which the vapour of water 

 may be replaced by another vapour, or even by a permanently 

 elastic fluid whose elasticity is augmented by heat. 



At the time when I entered upon these researches, the ques- 

 tion appeared to me more simple than it does at present. Adopt- 

 ing the views then recognised in science, it was easy to define 

 precisely the various data upon which it rested ; and I pictured 

 to myself the methods by the aid of which I hoped to be able 

 successively to discover the laws and determine the numerical 

 data. But, as is usually the case in sciences of observation, in 

 proportion as I advanced in my studies, the subject continually 

 expanded ; the questions which in the first instance appeared to 

 be very simple, now present a considerable degree of complexity ; 

 and perhaps I should not have had the courage to enter upon 

 the investigation of the subject, if I had from the first perceived 

 all the difficulties with which it is encompassed. 



Until very recently it was admitted that the quantities of heat 

 disengaged or absorbed by any one elastic fluid were always equal 

 when that fluid passed from a particular initial state to an iden- 

 tical final state, whatever might be the direction in which the 

 transition took place; in short, it was considered that these 



* From the Comptes Rendus, April 18th, 1853. 

 Phil Mag, S. 4. No. 35. Suppl. Vol. 5. 2 1 



