98 Mr. J. P. Joule on the Electric Origin 



Arabians as they took theirs from Ptolemy. The 7800 miles 

 which he assumed for the diameter of the earth might indeed 

 be taken as well from the reputed result of the Arabians as 

 from his own measure ; and having assumed this, he could 

 dispense with all the rest. But in whatever way he came by 

 his result, its near approximation to the true value of the degree 

 must be considered as a very remarkable ciixumstance. It is 

 singular that the historian of his life (Plantius), who was his 

 intimate friend, does not once alKide to the measure of the 

 earth, although it was a feat in which, as Lalande remarks, 

 Fernel might reasonably be expected to have prided himself*. 

 Fernel was court physician to Henry II. of France. In 

 his early years he cultivated mathematical studies, but aban- 

 doned them upon his marriage and devoted himself exclu- 

 sively to the practice of his profession, by which he acquired a 

 large fortune. He died in 1558, at the age of seventy-two. A 

 gossiping account of him may be seen in Bayle's Dictionary, 

 I am. Gentlemen, yours faithfully, 

 Serjeants' Inn, Jan. 6, 1842. T. GalLOWAY. 



XVII. On the Electric Origin of the Heat of Combustion. 



By J. P. Joule, Esq'^. 



[Illustrated by Plate III.] 



1. TN the papers which I had some time ago the honour of 



A communicating to the Royal Society, I related an in- 

 vestigation concerning the calorific effects of voltaic electricity, 

 and stated my opinion with regard to the heat evolved by com- 

 bustion and certain other chemical phsenomena. In the pre- 

 sent paper I intend to bring forward some experiments in 

 confirmation of my theory, and to prove that the heat of com- 

 bustion, terminating in the formation of an electrolyte, is the 

 consequence of resistance to electric conduction. 



2. We have seen that when those chemical actions which are 

 not the sources of transmitted electricity are allowed for, the heat 

 evolved from any part of the voltaic apparatus is the effect of 

 the I'esistance which is presented by that part to the electric 

 current ; and that hence it necessarily Ibllows, that the total 

 voltaic heat generated by the action ot any closed galvanic pair 

 is proportional to the number of chemical equivalents which 

 have been consumed in the act of propelling the current, and 

 the intensity of the galvanic arrangement. Now, if it can be 

 shown that the quantity of heat which is evolved by ordinary 



[* On the subject of this paper see an additional communication from 

 Professor de Morgan, at p. 116. — Edit.] 



t Read before the Literary and Piiilosophieal Society of Manchebter, 

 November 2, 1841 ; and now communicated by the Author. 



