of the Heat of Combustion. HI 



57. We shall now proceed to examine how far the theory 

 of resistance to electric conduction agrees with the above ex- 

 perimental results. 



58. We have seen (47.), (48.) and (49.) that the intensities 

 of the affinities which unite gaseous oxygen with zinc, iron, 

 potassium and gaseous hydrogen, areas 1-32, 1-18, 2*05 and 1 ; 

 and the proportional quantities of heat vv-hich were generated 

 by the combustion of the equivalents of these bodies are 10°-8, 

 9^-48, 17°'6, and 8°-36, or 1-29, 1-13, 2-105 and 1, a ratio 

 which is very nearly the same as that of the intensities just 

 given. Hence we see that the quantities of heat which are 

 evolved by the combustion of the equivalents of bodies are pro- 

 portional to the intensities of their ajfinities for oxygen. Now 

 I proved in my former paper* that a similar law obtains in 

 the voltaic apparatus, in consequence of its heat being pi'o- 

 duced by resistance to conduction. And hence we have an 

 argument that the heat of combustion has the same origin. 



bd. But our proof of the real character of the heat of com-> 

 bustion is rendered more complete by regarding quantities as 

 well as ratios of heat. From the quantity of heat generated by 

 the motion of a given current along a wire of known resist- 

 ance, we can deduce the quantities of heat which, accoi'ding to 

 the theory of resistance to electric conduction, ought to be 

 produced by the combustion of bodies ; and then these theo- 

 retical deductions may be compared with the results of expe- 

 riment, 



60. The mean of three careful experiments detailed in my 

 former paperf, shows that if a wire, the resistance of which is 

 an unit, be traversed by an electric current of l°-88 Q J for 

 one hour, the heat evolved by that wire will be able to increase 

 the temperature of a pound of water by 15°- 12. Now I have 

 ascertained experimentally that a pair consisting of amalga- 

 mated zinc and platinized silver, excited by dilute sulphuric 

 acid, is able to propel a current of 0°'168 Q against the whole 

 resistance of the circuit, when that resistance is 5'2 ; conse- 

 quently, a similar pair can propel a current of 0°-168 Q x 5'2 

 = 0°*874 Q against the resistance which I have called an unit. 

 But from (42.), the intensity necessary to separate oxide of zinc 

 into zinc and gaseous oxygen is to the intensity of one of 

 Smee's pairs as 3*7 : 1. Consequently, the electricity produced 



• I'liilosophical Magazine, October 1841, S. 3. vol. xix. p. 275. (70 ") 



+ Ibid. p. 266. 



X I beg to remind the reader that my degree, expressed thus (1°Q), in- 

 dicates that quantity nf current electricity which, after passing constantly 

 during one hour, is found to have elcctroiized a chemical equivalent ex- 

 pressed in grains; as, t) grains of water, aC grains of protoxide of iron, &c. 



