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 »?', 

 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 /i be the number of degrees by which w grains of 



TflXC 



water are heated, by the absorption of g grains of gas, — 



G 



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 diflaculties remained 

 to be overcome. How, it will be asked, is the true value 

 of n to be determined ? For the observation of temperature 



