I 



Prof. Thomson on the Dynamical Theory of Heat. 429 



researches afford actual tests, which, so far as they go, are veri- 

 fications of the truth of jNIayer's hyjiothesis for temperatures 

 between 50° and 60° Fahr., founded on two distinct methods, 

 either of which is perfect in principle, and might be made the 

 foundation of experiments at any temperature whatever. 



70. The first of these methods consists simply in determining, 

 by direct experiment, the heat evolved by the expenditure of a 

 given amount of work in compressing an*, and comparing it with 

 the quantity of heat created by the same amount of work in 

 Joule's original experiments on the heat developed by magneto- 

 electricity, and by the friction of fluids in motion. 



71. The second method is especially remarkable, as affording 

 in each experiment an independent test of the truth of Maye/s 

 hypothesis for air at the temperature used, without requiring any 

 knowledge of the absolute value of the mechanical equivalent of 

 heat. In Joule's actual experiments, the test is simply this : — 

 the total external thermal effect is determined when air is allowed 

 to expand, through a small orifice, from one vessel into another 

 previously exhausted by an air-pump. Here the first mechanical 

 effect produced by the expanding gas is vis viva generated in the 

 rushing of the air. By the time equilibrium is established, all 

 this mechanical effect has been lost in fluid friction (there being 

 no appreciable mechanical effect produced externally in sound, 

 which is the only external mechanical effect, other than heat, 

 that can be produced by the motions of a fluid within a fixed 

 rigid vessel) ; and no truth in physical science can be more cer- 

 tain, than that by the time thermal as well as mechanical equi- 

 librium is established at the primitive temperature, the contents 

 of the two vessels must have parted vviih just as much more heat 

 than they would have parted with had the air in expanding 

 pushed out a piston against an external resisting force, as is 

 equivalent to the mechanical effect thus produced externally. 

 Hence if the two vessels and the tube connecting them be im- 

 mersed (as they arc in Joule's first set of experiments with this 

 apparatus) in one vessel of water, and if, after time is allowed 

 for the pressure and temperature of the air to become the same 

 in the two vessels, the water be found to have neither gained nor 

 lost heat (it being understood, of course, that the air and all 

 other matter external to the water are at an absohitcly constant 

 temperature during the experiment), then, for the temperature 

 of the experiment, Mayei-'s hypothesis is perfectly confirmed ; 

 but any final elevation or depression of temperature in the water 

 would show that the work due to the expansion is either greater 

 than or less than the absolute equivalent of the heat absorbed. 



72. i\Ir. Joule's second expt'rimcnt on the same apparatus, in 

 which he examined separately the external thermal effects roiuid 



