Prof. Thomson on the Dynamical Theory of Heat. 17 



Let there be three equal and similar galvanic batteries fur- 

 nished with equal and similar electrodes ; let Aj and Bj be the 

 terminations of the electrodes (or wires connected with the two 

 poles) of the fost batteiy, Ag and B2 the terminations of the 

 corresponding electrodes of the second, and A3 and B3 of the 

 third battery. Let A, and Bj be connected with the extremities 

 of a long fixed wire ; Jet Ag and Bg be connected with the "poles" 

 of an electrolytic apparatus for the decomposition of water ; and 

 let A3 and Bg be connected with the poles (or ports as they might 

 be called) of an electro-magnetic engine. Then if the length of 

 the wire between Aj and B,, and the speed of the engine between 

 A3 and Bg, be so adjusted that the strength of the current (which 

 for simplicity we may suppose to be continuous and perfectly 

 uniform in each case) may be the same in the three circuits, there 

 will be more heat given out in any time in the wii-e between Aj 

 and B, than in the electrolytic apparatus been Ag and Bg, or the 

 working engine between A3 and Bg. But if the hydrogen were 

 allowed to burn in the oxygen, within the electrolytic vessel, and 

 the engine to waste all its work without producing any other 

 than thermal effects (as it would do, for instance, if all its work 

 were spent in continuously agitating a limited fluid mass), the 

 total heat emitted would be precisely the same in each of these 

 two pieces of apparatus as in the wire between A, and Bp It is 

 worthy of remark that these propositions are rigorously true, 

 being demonstrable consequences of the fundamental principle 

 of the dynamical theoiy of heat, which have been discovered by 

 Joule, and illustrated and verified most copiously in his experi- 

 mental researches*. 



19. Both the fundamental propositions may be applied in a 



of Electrolysis " (published in vol. vii. part 2, of the second Series of the 

 Literarj' and Philosophical Society of Manchester) for experimental de- 

 monstration of those parts of the theory in which chemical action is con- 

 cerned.] 



* [I have recently met with the following passage in Liebig's Animal 

 Chemistry (3rd edit. London, 1846, p. 43), in which the dynamical theory of 

 the heat both of combustion and of the galvanic battery is indicated, if not 

 fully expressed : — " When we kindle a fire under a steam-engine, and em- 

 ploy the power obtained to produce heat by friction, it is impossible that 

 the heat thus obtained can ever be greater than that which was required to 

 heat the boiler ; and if we use the galvanic current to produce heat, the 

 amount of heat obtained is never in any circumstances greater than we might 

 have by the combustion of the zinc which has been dissolved in the acid." 



A paper " On the Heat of Chemical Combination," by Dr. Thomas 

 Woods, published last October in the Philosophical Magazine, contains an 

 indejjendent and direct experimental demonstration of the proposition 

 stated in the text regarding the comparative thermal effects in a fixed nie- 

 talhe wire, and an electrolytic vessel for tlie decomposition of water, pro- 

 duced by a galvanic current. — W. T. March 20, 1852.] 



Phil May, S. 4. Vol. 4. No. 23. July 1852. C 



