494 Mr. J. P. Joule on the Heat disengaged 



and carefully observe the quantity of decomposition and the heat 

 evolved. By the law of Ohm I then ascertain the resistance of 

 a wire capable of obstructing the current equally with the elec- 

 trolytic cell. Then, by the law we have proved, I determine the 

 quantity of heat which would have been evolved had a wire of 

 such resistance been placed in the circuit instead of the electro- 

 lytic 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 two 

 results evidently gives the quantity of heat absorbed during the 

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

 to the reverse chemical combination by combustion or other 

 means. 



Having thus given a short outline of the process, I shall at 

 once proceed to describe the experiments in detail. 



1st. Heat evolved by the Combustion of Copper. 



I took a glass jar, fig. 6, filled with 3 lbs. of a solution con- 

 sisting of 24 parts of water, 7 parts of crystallized sulphate of 

 copper, and 1 part of strong sulphuric acid. In this solution, 

 two plates, one of platinum, the other of copper, were immersed, 

 each being connected by means of a proper clamp with a thick 

 copper wire passing through a cork in the mouth of the vessel, 

 and terminating in a mercury cup a. A very delicate thermo- 

 meter, each of whose divisions was equal to ^^ of a degree Cent., 

 was also fixed in the cork so as to have its bulb nearly in the 

 centre of the liquid. Lastly, a glass stirrer b was introduced. 



The experiments were conducted in the following manner : — A 

 pile consisting of four large cells of Daniell («, fig. 7) was con- 

 nected with the galvanometer b by means of two thick copper 

 wires, one of which was continuous, while the other was divided 

 at the mercury cups cc. The connexion between these mercury 

 cups was first established by means of a short thick copper wire, 

 and the deflection of the needle noted. The quantity of current 

 indicated by this deflection I shall call A. The thick copper 

 wire was now removed from the cups at cc, and the standard 

 coil of silver wire (immersed in water to keep it cool) was put 

 thei'e instead, and the deflection again noted. The current ob- 

 served in this second instance I shall call B. The coil of silver 

 wire was now removed, and the electrolytic cell above described 

 being put in its stead, electrolysis was carried on for exactly 10' 

 of time, during which the deflections of the needle were noted at 

 equal intervals of time. The current indicated by the mean of 

 these observations I shall call C. Currents B and A were then 

 again observed in the reverse order ; and the mean of these and 



