108 



MR. J. P. JOULE ON HEAl", 



either the vis viva of ponderable particles, or a state of 

 attraction or repulsion capable of generating vis viva. 



It now became important to ascertain the mechanical 

 equivalent of heat, with as much accuracy as its importance 

 to physical science demanded. For this purpose the mag- 

 netic apparatus was not very well adapted ; and therefore I 

 sought in the heat generated by the friction of fluids for 

 the means of obtaining exact results. I found, firstly; that 

 the expenditure of a certain amount of mechanical power 

 in the agitation of a given fluid, uniformly produced a cer- 

 tain fixed quantity of heat ; and, secondly, that the quan- 

 tity of heat evolved in the friction of fluids was entirely 

 uninfluenced by the nature of the liquid employed, for 

 water, oil, and mercury, fluids as diverse from one another 

 as could have been well selected, gave sensibly the same 

 result; viz., that the quantity of heat capable of raising the 

 temperature of a lb. of water 1°, is equal to the mechanical 

 power developed by a weight of 770 lbs. in falling through 

 one perpendicular foot.* 



Believing that the discovery of the equivalent of heat 

 furnished the means of solving several interesting pheno- 

 mena, I commenced, in the spring of 1844, some experi- 

 ments on the changes of temperature occasioned by the 

 rarefaction and compression of atmospheric air.f It had 

 long been known that air, when forcibly compressed, 

 evolves heat; and that, on the contrary, when air is dilated, 

 heat is absorbed. In order to account for these facts, it was 

 assumed that a given weight of air has a smaller capacity 

 for heat when compressed into a small compass than when 

 occupying a larger space. A few experiments served to 

 show the incorrectness of this hypothesis : thus, I found 

 that by forcing 2956 cubic inches of air, at the ordinary 



• The equivalent I have since arrived at is 772 foot pounds. See 

 Phil. Trans. 1850, Part I.— May, 1851. J. P. J. 

 t Philosophical Magazine, vol. xxvi. 



