ЗЕСТ. II.]
PROPERTIES OF STEAM.
51
tain not only its own 188°, but also the 188° lost by each of the other four parts;
that is to say, it must contain 188° x 5, or about 940° of heat.
76. The experiments of Dr. Black are not greatly different from the result
obtained by Schmidt, for the latter found the heat of steam to be 5*33 times the
heat which is required to boil water of the temperature 32°, the barometer being
at 29*84 inches. 1 This is the best mode of expressing the heat, for there is reason
to believe that the specific heat of water is not the same for every rise of tempe
rature. But to reduce it to the usual measure in degrees, there are 180° between
the boiling and freezing point, hence 180 x 5*33=959°*4 for the additional heat of
steam.
77. Mr. Southern, and Mr. W. Creighton in 1803, made some experiments
by condensing steam with a considerable degree of care; the steam being gene
rated at different temperatures and pressures. The pressure, temperature, heat of
formation, and bulk of the steam, from a cubic inch of water, are shown in the
following table :
Pressure in inches
of mercury.
Temperature.
Heat required to
form the steam.
Bulk of steam
from one cubic
inch of water
at 60°.
Bulk calculated
from the first ex
periment.
40
229°
1157°
208
1208
80
270
1244
588
635
120
295
1256
404
427
If from the whole heat we deduct the difference of temperature, we have 1157°,
1203°, and 1190°; whence it appears that the heat to form steam is nearly a
constant quantity when the temperature is the same, being independent of the
density.
Therefore the most convenient mode of expressing the quantity of heat is that
adopted by Mr. Southern, which consists in ascertaining the constant quantity of
heat required to be added to the actual temperature of the steam to give the whole
heat necessary to form it. This quantity is
1157—229=928°; 1244-270=974°; and 1256-295=961°; and the mean
is 954°.
In another set of experiments, made under the same pressures and tempera
tures, the quantities of heat required in addition to the temperature were 942°, 942°
and 950°, 2 the mean being nearly 945°, and the mean of both sets 949°. In this
set of experiments an allowance was made for the heat communicated to the
vessel; in the former set none was made.
1 Nicholson’s Philosophical Journal, vol. v. p. 208. octavo series.
2 Robison’s Meehan. Phil. vol. ii. p. 160—166.