52
THE NATURE AND
[sect. II.
These experiments are valuable, because they afford a proof that the additional
heat required for steam is either accurately or nearly a constant quaritity. 1
78. And they also show that the bulk or volume of steam is inversely as the
pressure, when the temperature is not altered. For as 80 : 40 : : 1208 : 604, which
added to the expansion would be 635, nearly; and 120 : 40 : : 1208 : 402, and
adding the expansion it is 427, nearly; and conversely the density is directly
as the pressure; the experiments being quite as near as could be expected in so
extremely delicate an operation.
79. Count Rumford obtained a higher result; and from his known skill in
such inquiries, much confidence may be placed in his experiments. The heat
was measured by means of the temperature communicated to a copper vessel
filled with water, which he called his calorimeter. Within this calorimeter a thin
serpentine pipe of copper contained the steam to be condensed; hence the fluids
did not mix together, and loss by the escape of vapour was prevented.
The water which the calorimeter contained was of a lower temperature than that
of the room by 5° or 6°; and when the thermometer of the calorimeter announced
an augmentation of temperature of 10° or 12°, an end was put to the experiment.
The water produced by the condensation of the vapour in the serpentine was
carefully weighed, and from its quantity, as well as from the heat communicated
to the calorimeter, the heat developed by the vapour in its condensation was deter
mined.
As a small part of the heat communicated to the calorimeter was produced
from the cooling of the water, condensed in the serpentine pipe after the vapour
had been changed into water, an account was kept of this heat. It was supposed
that the water at the moment of condensation was at the temperature of 212°,
being that of boiling water ; and it was determined by calculation, what part of
the heat communicated to the calorimeter must have been owing to the boiling-
water.
In making this calculation, Count Rumford remarks, no “ account was taken of
the difference in the capacity of water for heat, which depends on its temperature:
this is but imperfectly known ; and besides, the correction which would have been
the result could not but have been very small.”
The following are the details and results of two experiments made on the 21st,
of January, 1812. The duration of each of the two experiments was from ten to
eleven minutes. The water had been boiled for some time to drive out the air
which it contained, before the steam was directed into the serpentine pipe of the
calorimeter.
1 M. Despretz, Ann. de Chim. et de Pliys. xxiv. 329. makes it 955°-8.