in Chemical Combinations. 



493 



riments was carefully guarded against by a repetition of each 

 experiment in the reverse order ; i. e. beginning with current C 

 and ending with current A, and then taking the mean of the 

 two sets of observations. 



Table IV. 



On multiplying 0-74964 by 2-7397, the square of the inten- 

 sity of the current to wbich the inei-cury wire was exposed (see 

 Table III.), we obtain 2'0538, a quantity which ought to be 

 proportional to the heat evolved, if our law be correct. From 

 Table III. we see also, that, in the case of the silver wire, the 

 square of the current multiplied by its resistance (which we called 

 unity) is 2 - 131, while the heat evolved was 41 '425. Hence we 

 have for the heat which ought to have been evolved by the mer- 

 cury spiral, 



|^x 41-425 = 39-924. 



Referring again to Table III., we find that the heat actually 

 evolved was 39'69. The difference between this number and 

 the result of theory, trifling as it is, is almost entirely accounted 

 for by the circumstance, that the capacity for heat of the mercury 

 spiral exceeded that of the coil of silver wire by a quantity equal 

 to the capacity of 5-64 grms. of water. Hence we must apply a 

 correction of ^^ to the observations with the mercurial appa- 

 ratus. This brings the heat actually evolved up to 39-868, a 

 quantity differing from 39-924, the theoretical result, only by 

 0-056 of a division of the thermometer, or o- 0024 of a degree 

 Centigrade. 



7. Having thus given fresh proofs of the accuracy of the law 

 of the evolution of heat by voltaic electricity, we may now pro- 

 ceed to apply it in order to determine the quantity of heat evolved 

 in chemical combinations. The following is an outline of my 

 process: — I take a glass vessel filled with the solution of an elec- 

 trolyte, and properly furnished with electrodes. I place this 

 electrolytic cell in the voltaic circuit for a given length of time, 



