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.