442 Mr. Joule on the Mechanical Value of Heat. 



the voltaic circuit into useful mechanical power, it is evident 

 that the electro-magnetic engine, worked by the voltaic bat- 

 teries at present used, will never supersede steam in an ceco- 

 nomical point of view. 

 Broom Hill, Pendlebury, 

 near Manchester, July 1843. 



P.S. — We shall be obliged, after all, to admit that Count 

 Rumford was right in attributing the heat evolved by boring 

 cannon to friction, and not (in any considerable degree) to 

 any change in the capacity of the metal. I have myself proved 

 experimentally that heat is evofoed by the passage of water 

 through narrow tubes. My apparatus consisted of a piston 

 perforated by a number of small holes, working in a cylindrical 

 glass jar containing about 7 lbs. of water. I thus obtained 

 one degree of heat per lb. of water from a mechanical force 

 capable of raising about 770 lbs. to the height of one foot, — 

 a result which will be allowed to be very strongly confirma- 

 tory of our previous deductions. I shall lose no time in re- 

 peating and extending these experiments, being satisfied that 

 the grand agents of nature are, by the Creator's fiat, inde- 

 structible', and that wherever mechanical force is expended, 

 an exact equivalent of heat is always obtained. 



On conversing a few days ago with my friend Mr. John 

 Davies, he told me that he had himself, a few years ago, at- 

 tempted to account for that part of animal heat which Craw- 

 ford's theory had left unexplained, by the friction of the blood 

 in the veins and arteries, but that, finding a similar hypothesis 

 in Haller's 'Physiology*,' he had not pursued the subject 

 further. It is unquestionable that heat is produced by such 

 friction, but it must be understood that the mechanical force 

 expended in the friction is a part of the force of affinity which 

 causes the venous blood to unite with oxygen ; so that the 

 whole heat of the system must still be referred to the chemical 

 changes. But if the animal were engaged in turning a piece 

 of machinery, or in ascending a mountain, I apprehend that 

 in proportion to the muscular effort put forth for the purpose, 

 a diminution of the heat evolved in the system by a given 

 chemical action would be experienced. 



I will observe in conclusion, that the experiments detailed in 

 the present paper do not militate against, though they certainly 

 somewhat modify the views I had previously entertained with 

 respect to the electrical origin of chemical heat. I had before 

 endeavoured to prove that when two atoms combine together, 

 the heat evolved is exactly that which would have been evolved 

 by the electrical current due to the chemical action taking 

 place, and is therefore proportional to the intensity of the 

 * Haller's Physiology, vol. ii. p. 304. 



