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.