212 Mr. J. P. Joule on Heat, 
derable particles, or a state of attraction or repulsion capable of 
generating vis viva. 
It now became important to ascertain the mechanical equiva- 
lent of heat with as much accuracy as its importance to physical 
science demanded. For this purpose the magnetic 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, first, that the expenditure of a certain 
amount of mechanical power in the agitation of a given fluid 
uniformly produced a certain fixed quantity of heat; and, secondly, 
that the quantity 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 
pound 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 phenomena, I com- 
menced, in the spring of 1844, some experiments on the changes 
of temperature occasioned by the rarefaction and compression of 
atmospheric airt. It had long been known that air, when for- 
cibly 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 atmospheric 
pressure, into the space of 136} cubic inches, 13°63 of heat per 
pound of water were produced ; whereas by the reverse process, 
of allowing the compressed air to expand from a stopcock into 
the atmosphere, only 4°-09 were absorbed instead of 13°63, 
which is the quantity of heat which ought to have been absorbed, 
according to the generally received hypothesis. I found, also, 
that when strongly compressed air was allowed to escape into a 
vacuum, no cooling effect took place on the whole, a fact like- 
wise at variance with the received hypothesis. On the contrary, 
the theory I ventured to advocatet was in perfect agreement 
with the phenomena; for the heat evolved by compressing the 
* The equivalent I have since arrived at is 772 foot-pounds. See Phil. 
Trans. 1850, part 1.—May 1851, J. P. J. 
+ Phil. Mag. vol. xxvi. 
t I subsequently found that M. Mayer had previously advocated a similar 
hypothesis, without, however, attempting an experimental demonstration of 
its aecuracy.—Annalen of Wohler and Liebig for 1842.—May 1851, J.P. J. 
= 
