370 Mr. Joule ofi tJie Changes of Teinperattire produced by 



evolved when air is compressed to one-half of its original 

 bulk ; and that, on the other hand, 50° are absorbed by a cor- 

 responding rarefaction*. 



There is every reason for believing that Dalton's results are 

 very near the truth, especially as they have been exactly con- 

 firmed by the experiments of Dr. Ure with the thermometer 

 of Breguet. But our knowledge of the specific heat of elastic 

 fluids is of such an uncertain character, that we should not be 

 justified in attempting to deduce from them the absolute quan- 

 tity of heat evolved or absorbed. I have succeeded in re- 

 moving this difficulty by immersing my condensing pumj) and 

 receiver into a large quantity of water, so as to transfer the 

 calorific effect to a body which is universally received as the 

 standard of capacity. 



My apparatus will be understood on inspecting fig. 1 . C 



Fig. 1. 



* Memoirs of tlie Literary and PhiIosoi)hical Society of Manchester, 

 vol. V. part 2. p. 521-525. 



