54
THE NATURE AND
[sect. II.
On determining, by calculation, the quantity of water which may be heated
one degree, by the heat developed in the condensation of the vapour, he took care
to keep an account of the difference of the capacity of water for heat from that of
alcohol. 1
The result of Count Rumford’s calculation is nearly the same as by the formula,
(art. 75, note,) when we assume the specific heat of the alcohol vapour and liquid
to be the same, and equal to 58. Thus from the second experiment
42909 X 10
755
+ (-58 X 65-5) = G06°-3,
from whence, deducting 173 x *58 for the heat due to the temperature of the
vapour, we have 506° nearly, for the heat of conversion from liquid to vapour.
The Count’s number is 50003.
Count Rumford also ascertained that the vapour of sulphuric ether afforded
only about half the heat in condensation that alcohol afforded, or one-fourth of
the heat furnished by condensing the steam of water.
81. Important as a knowledge of the heat of conversion into vapour is, it was
not further investigated till 1817, when Dr. Ure made a few experiments on diffe
rent bodies. 2 His mode of experiment was exceedingly simple. The apparatus
consisted of a glass retort of very small dimensions, with a short neck, inserted
into a globular receiver, of very thin glass, about three inches in diameter. The
glass was fixed steadily in the centre of 32340 grains of water, at a known tempe
rature, contained in a glass basin. Of the liquid, whose vapour was to be
examined, 200 grains were introduced into the retort, and rapidly distilled into the
globe by the heat of an Argand lamp. The temperature of the air was 45°, that
of the water in the basin from 42° to 43°; and the rise of temperature occasioned
by the condensation of the vapour never exceeded that of the air by four degrees.
As the communication of heat is very slow between bodies which differ little in
temperature, the air could exercise no perceptible influence on the water in the
basin during the experiment, which was always completed in five or six minutes.
A thermometer of great delicacy was continually moved through the water, and its
indications were read off, by the aid of a lens, to small fractions of a degree.
The distillation was rapidly performed; and we are assured by Dr. Ure, that in
the numerous repetitions of the same experiment, the accordances were excellent.
The following table gives the mean result, the last column being calculated by the
formula, (of note to art. 75.)
1 Philosophical Mag. voi. xliii. p. 67.
2 Philosophical Transactions for 1818.