238 Dr. C. R. A. Wright on the Determination of 



to that evolved by the recombination of the hydrogen and 

 oxygen evolved. 



2. Joule's method of operating is described by him in the 

 following words : — " I take a glass vessel filled with the solu- 

 tion of an electrolyte and properly furnished with electrodes ; 

 I place the electrolytic cell in the voltaic circuit for a given 

 length of time, and carefully observe the quantity of decompo- 

 sition and the heat evolved. By the law of Ohm I then ascer- 

 tain the resistance of a wire capable of obstructing the current 

 equally with the electrolytic cell. Then by the law we have 

 proved [i. e. ' Joule's law'] I determine the quantity of heat 

 which would have been evolved had a wire of such resistance 

 been placed in the current instead of the electrolytic cell: this 

 theoretical quantity, being compared with the heat actually 

 evolved in the electrolytic cell, is always found to exceed the 

 latter considerably. The difference between the results evi- 

 dently gives the quantity of heat absorbed during the electro- 

 lysis, and is therefore equivalent to the heat which is due to 

 the reverse chemical combination by combustion or other 

 means " (loc. cit. pp. 493, 494). 



Put into symbols this may be thus expressed : — Let h x be 

 the heat actually developed in the electrolyte by a current Cx 

 in a given time t during which iu x grammes of electrolyte are 

 decomposed. Let h 2 be the heat that would be developed in a 

 wire of the same resistance (i. e. capable of equally "obstruct- 

 ing the current ") in the same time t by the same current Ci. 

 Let A 3 be the heat actually developed in another wire by a 

 different current C 2 in the same time t ; and let the resistance 

 of the electrolytic cell be a times the resistance of this second 

 wire. Then, by Joule's law, 



V c* ' 



whence the heat absorbed in decomposing w grammes of elec- 

 trolyte is 



C 2 

 h—hi or ==:h 3 -±a—k l9 



and the heat absorbed in decomposing 1 gramme of electro- 

 lyte is 



C 2 



10 



In this way Joule obtained in three sets of observations the 

 following mean values for the heat absorbed in the decompo- 



