JAMES PRESCOTT JOULE 183
was exhausted. When the temperature of the water had
become uniform and constant, the stop-cock was opened,
and the air allowed to rush from the filled into the empty
receiver. The tank water was stirred and its temperature
again measured. No nett change in temperature could be
observed, so Joule deduced that "no change of temperature
occurs when air is allowed to expand in such a manner as not
to develop mechanical power "
He repeated the experiments with the two copper
receivers and the connecting tube in three separate water
tanks. He found that 2-36° of cold was produced in the
water surrounding the receiver which had contained air at
pressure, and 2-38° was produced in the previously empty
receiver, while 0-31° appeared in the water surrounding,
the stop-cock tube. After allowance for conduction of heat
into the cold receiver, these figures satisfactorily balanced,
One of the copper receivers, containing air at high
pressure, had a spiral leaden pipe attached to the stop-cock.
The receiver and spiral were put in a water tank, and the air
allowed to escape through the pipe into a jar by which its
volume could be measured. The cold produced was equal
to the quantity required to decrease the temperature of
i Ib. of water by 4-085° F, The work needed to compress
the gas was 3,352 foot-pounds, giving 820 as the mechanical
equivalent of heat.
Joule writes that these results are inexplicable on the
assumption that heat is a substance, but are "such as might
have been deduced a -priori from any theory in which heat
is regarded as a state of motion among the constituent
particles of bodies."
He concludes the paper by commenting that his results
appear to be in contradiction with the views of Carnot and
Clapeyron, who suppose no heat is lost in the working of
the steam-engine. "The theory here advanced demands
that the heat given out in the condenser shall be less than
that communicated to the boiler from the furnace, in
exact proportion to the equivalent of mechanical power
developed."
By these experiments Joule proved to a high accuracy