THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



JULY 1868. 



I. On the Dynamical Theory of Heat. 

 By Joseph Gtll, Esq.* 



THE first law of thermodynamics enounces that heat and 

 mechanical energy are mutually convertible. The direct 

 conversion of mechanical energy into heat seems sufficiently ob- 

 vious from various known phenomena of friction and percussion ; 

 and the admirable experiments of Joule have shown, with an 

 accuracy sufficient for all practical purposes, that in the process 

 of friction mechanical energy is directly converted or transformed 

 into heat in the proportion of 772 foot-pounds for each British 

 unit of heat. The direct conversion of heat into work is not so 

 obvious, and satisfactory experimental proofs of the fact are still 

 wanting. 



Physically considered, the universe has been defined as matter 

 and motion; and, as far as human means are concerned, it is 

 allowed that matter and motion can neither be created nor de- 

 stroyed. In all the wonderful transformations of matter revealed 

 by chemistry, it is well known that the quantities of the sub- 

 stances acted on are absolutely constant ; not an atom is de- 

 stroyed or created ; and physical science now demonstrates in a 

 manner almost equally convincing that motion, or the free un- 

 balanced action of force on matter, as well as force itself in a 

 potential state, where it is counterbalanced by some equivalent 

 resistance, is also absolutely constant in all the various transfor- 

 mations through which it may be caused to pass. The genius of 

 Lavoisier established the truth of the conservation of matter ; and 

 many living authorities of the highest scientific standing have 

 concurred in establishing the certainty of the conservation of 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 36. No. 240. July 1868. B 



