HELMHOLTZ ON THE CONSERVATION OF FORCE. 135 



states of aggregation brought about, and thus heat rendered 

 latent. Whether by the development of mechanical force heat 

 disappears, which would be a necessary postulate of the conser- 

 vation of force, nobody has troubled himself to inquire. I can 

 only in respect to this cite an experiment by Joule*, which 

 seems to have been carefully made. He found that air while 

 streaming from a reservoir with a capacity of 136*5 cubic inches, 

 in which it was subjected to a pressure of 22 atmospheres, cooled 

 the surrounding water 4°'085 Fahr. when the air issued into the 

 atmosphere, and therefore had to overcome the resistance of the 

 latter. When, on the contrary, the air rushed into a vessel of 

 equal size which had been exhausted of air, thus finding no 

 resistance and exerting no mechanical force, no change of tem- 

 perature took place. 



We have now to examine the manner in which the views of 

 Clapeyront and Holtzmann t bear upon our own. Clapeyron 

 starts from the notion that only by its passage from warm bodies 

 to cold ones can heat be applied as a means of developing me- 

 chanical force, and that the maximum of the latter is gained when 

 the transmission of the heat is effected between bodies of equal 

 temperatures, the alterations of temperature being effected by 

 the compression and dilatation of the heated bodies. This 

 maximum must be the same for all natural bodies which by 

 heating and cooling can produce mechanical force; for were 

 it different, that body in which a certain quantity of heat was 

 capable of producing the greatest action might be applied in 

 the production of mechanical work, and a portion of the latter 

 might be made use of to bring the heat back from the colder 

 to the warmer source, and thus mechanical force to infinity 

 might be gained, it being at the same time tacitly assumed 

 that the quantity of heat cannot be changed by this process. 

 The following is the analytical expression given by Clapeyron 

 to this law : — 



dq dt ^dq dt _^^ 

 dv dp dp' dv 



* Philosophical Magazine, S. 3. vol. xxvi. p. 369. 

 t Scientific Memoirs, vol. i. Part 3 of 1st Series. 



I Ueher die Wdrme und Elasticitdt der Gase und Ddmpfe. Manheim, 1845. 

 Translated in full in vol. iv. Part 14., of Scientific Memoirs. 



L 2 



