436 
of which the gas, when dried, could be conveyed into a cup 
containing mercury covered with a stratum of water, and, by 
the other, into a small cylinder of very thin copper containing 
the liquid to be heated. The cylinder was furnished with two 
necks, into one of which a very delicate thermometer was fitted 
by means of a cork, while the second received the tube by 
which the gas was to be introduced. By turning a stopcock 
attached to the latter tube, the gas could at any instant be 
conveyed into the copper cylinder, or excluded from it, an 
assistant at the same moment raising or depressing the vessel 
containing the mercury, so as to prevent or permit of the issue 
of the gas at this part of the apparatus. The temperature of 
the air at the instant of the admission of the gas, and the tem- 
perature of the liquid in the cylinder, having been accurately 
noted, and, in addition, the time m, which elapsed between the 
introduction and exclusion of the gas, as also the time m’, 
which intervened between the latter manipulation and the 
second reading of the thermometer, data were obtained for cal- 
culating the total rise of temperature, and hence, for estimating 
the heat evolved by the gas as a consequence of its absorption. 
Thus, if 2 be the number of degrees by which w grains of 
water are heated, by the absorption of G grains of gas, a 
will be the heat extricated, i. e., the number of degrees that 
the caloric given out by the gas would heat an equal weight 
of water. 
‘¢ Such is an outline of the general course pursued. The 
experiments required the greatest attention, with a view to the 
management of the apparatus, and the accurate mensuration 
of time and temperature; and I am not a little indebted to my 
scientific friend, Dr. Head, for the valuable assistance which 
he has rendered to me. 
‘*¢ The observations being made, other difficulties remained 
to be overcome. How, it will be asked, is the true value 
of n to be determined ? For the observation of temperature 
=e 
