THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 

 MAY 1845. 



LIV. On the Changes of Temperature produced by the Rare- 

 faction and Condensation of Air. By J. P. Joule, Esq.* 



TN a paperf which was read before the Chemical Section of 

 ■*- the British Association at Cork, I applied Dr. Faraday's 

 fine discovery of magneto-electricity in order to establish defi- 

 nite relations between heat and the ordinary forms of mecha- 

 nical power. In that paper it was demonstrated experiment- 

 ally that the mechanical power exerted in turning a magneto- 

 electrical machine is converted info the heat evolved by the 

 passage of the currents of induction through its coils ; and, on 

 the other hand, that the motive power of the electro-magnetic 

 engine is obtained at the expense of the heat due to the che- 

 mical reactions of the battery by which it is worked. I hope, 

 at a future period, to be able to communicate some new and 

 very delicate experinients, in order to ascertain the mechani- 

 cal equivalent of heat with the accuracy which its importance 

 to physical science demands. My present object is to relate 

 an investigation in which I believe I have succeeded in suc- 

 cessfully applying the principles before maintained to the 

 changes of temperature arising from the alteration of the den- 

 sity of gaseous bodies, — an inquiry of great interest in a prac- 

 tical as well as theoretical point of view, owing to its bearing 

 upon the theory of the steam-engine. 



Dr. CuUen and Dr. Darwin appear to have been the first 

 who observed that the temperature of air is decreased by rare- 

 faction and increased by condensation. Other philosophers 

 have subsequently directed their attention to the subject. 

 Dalton was, however, the first who succeeded in measuring 

 the change of temperature with some degree of accuracy. By 

 the employment of an exceedingly ingenious contrivance, that 

 illustrious philosopher ascertained that about 50° of heat are 

 * Comnuuiicated by the Author, 

 t Phil. Mag. S. 3. vol. xxiii. pp. 263, 347, 435. 



Phil. Mag. S. 3. Vol. 26. No. 174. May 1845. 2 C 



