xMR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
63 
enabled him to advance the idea that the so-called imponderable bodies are merely 
the exponents of different forms of Force. Mr. Grove and M. Mayer have also 
given their powerful advocacy to similar views. 
My own experiments in reference to the subject were commenced in 1840, in which 
year I communicated to the Royal Society my discovery of tlie law of the heat evolved 
by voltaic electricity, a law from which the immediate deductions were drawn, — 1st, 
that the heat evolved by any voltaic pair is proportional, cceteris paribus, to its inten- 
sity or electromotive foree*; and 2nd, that the heat evolved by the combustion of 
a body is proportional to the intensity of its affinity for oxygen-f'. I thus succeeded 
in establishing relations between heat and chemical affinity. In 1843 I showed that 
the heat evolved by magneto-electricity is proportional to the force absorbed ; and 
that the force of the electro-magnetic engine is derived from the force of chemical 
affinity in the battery, a force which otherwise would be evolved in the form of heat: 
from these facts I considered myself justified in announcing “that the quantity of 
heat capable of increasing the temperature of a lb. of water by one degree of Fahr- 
enheit’s scale, is equal to, and may be converted into, a mechanical force capable 
of raising 838 lbs. to the perpendicular height of one foot:|;.” 
In a subsequent paper, read before the Royal Society in 1844, I endeavoured to 
show that the heat absorbed and evolved by the rarefaction and condensation of air 
is proportional to the force evolved and absorbed in those operations §. The quan- 
titative relation between force and heat deduced from these experiments, is almost 
identical with that derived from the electro-magnetic experiments just referred to, 
and is confirmed by the experiments of M. Seguin on the dilatation of steam ||. 
From the explanation given by Count Rumford of the heat arising from the fric- 
tion of solids, one might have anticipated, as a matter of course, that the evolution 
of heat would also be detected in the friction of liquid and gaseous bodies. More- 
over there were many facts, such as, for instance, the warmth of the sea after a few 
days of stormy weather, which had long been commonly attributed to fluid friction. 
Nevertheless the scientific world, preoccupied with the hypothesis that heat is a sub- 
stance, and following the deductions drawn by Pictet from experiments not suffi- 
ciently delicate, have almost unanimously denied the possibility of generating heat in 
that way. The first mention, so far as I am aware, of experiments in whieh the evo- 
lution of heat from fluid friction is asserted, was in 1842 by M. Mayer^, who states 
that he has raised the temperature of water from 12° C. to 13° C., by agitating it, 
without however indicating the quantity of force employed, or the precautions taken 
to secure a correct result. In 1843 I announced the fact that “heat is evolved by 
the passage of water through narrow tubes**,” and that each degree of heat per lb. 
of water required for its evolution in this way a mechanical force represented by 
* Phil. Mag. vol. xix. p. 275. f Ibid. vol. xx. p. 111. , J Ibid. vol. xxiii. p. 441. 
§ Ibid, vol.xxvi. pp. 375. 379. || Comptes Rendus, t. 25, p. 421. 
^ Annalen of Wcehler and Liebig, May 1842. Phil. Mag. vol. xxiii. p. 442. 
