334 
MR. J. P. JOULE AND PROFESSOR THOMSON ON THE 
Temperature of 
the bath. 
Excess of pressure, 
P-P'. 
Cooling effect, 
S. 
Cooling effect divided by 
excess of pressure. 
From Table 
IX 
18-962 
5-958 
0-459 
•0770 
From Table VIII. .. — 
20-001 
18-590 
1-446 
•0778 
From Table 
X 
19-077 
37-248 
2-938 
•0789 
From Table 
XI 
12-844 
60-601 
5-049 
•0833 
Mean 17421 
Mean of first three "0779 
Mean of all -0793 
We shall see immediately that the temperature of the bath makes a very eonsider- 
able alteration in the cooling effect, and we therefore select the first three results, 
obtained at nearly the same temperature, in order to indicate the effect of pressure. 
On referring to Chart No. 3, it will be remarked that these three results range them- 
selves almost accurately in a straight line. Or, by looking to the numbers in the 
last column, we arrive at the same conclusion. 
Cooling Effect experienced by Hydrogen in passing through a porous Plug. 
Not having been able as yet to arrange the large apparatus so as to avoid danger 
in using this gas in it, we have contented ourselves for the present with obtaining a 
determination by the help of the smaller force-pump employed in our preliminary 
experiments. The hydrogen, after passing through a tube filled with fragments of 
caustic potash, was forced, at a pressure of 68’4 lbs. on the inch, through a piece of 
leather in contact with the bulb of a small thermometer, the latter being protected 
from the water of the bath by a piece of india-rubber tube. At a temperature of 
about 10° Cent., a slight cooling effect was observed, which was found by repeated 
trials to be 0°*076. The pressure of the atmosphere being 14*7 lbs., it would appear 
that the cooling effect experienced by this gas is only one-thirteenth of that observed 
with atmospheric air. We state this result with some reserve, on account of the 
imperfection of such experiments on a small scale, but there can be no doubt that 
the effect of hydrogen is vastly inferior to that of atmospheric air. 
Influence of Temperature on the Cooling Effect. 
By passing steam through pipes plunged into the water of the bath, we were able 
to maintain it at a high temperature without any considerable variation. The passage 
of hot air speedily raised the temperature of the stem of the thermometer, as well as 
of the glass tube in which it was enclosed ; but nevertheless the precaution was taken 
of enclosing the whole in a tin vessel, by means of which water in constant circula- 
tion with the water of the bath was kept within one or two inches of the level of the 
mercury in the thermometer. The bath was completely covered with a wooden lid, 
and the water kept in constant and vigorous agitation by a proper stirrer. 
