

in Chemical Combinations. 491 



By comparing the last two columns of the foregoing table 

 with each other, we see that throughout a veiy extensive range 

 of electric intensities the heat evolved in a given time remains 

 proportional to the square of the quantity of transmitted elec- 

 tricity. 



6. Having thus succeeded in giving another proof of the law 

 of voltaic heat as far as regards a change in the intensity of the 

 current, we may now proceed to consider the effects produced 

 by a change in the resistance of the wire. It will not be neces- 

 sary for me to enter very largely upon this part of the subject, 

 inasmuch as it has long been admitted by philosophers that the 

 heat evolved by a cm-rent of given intensity is proportional to 

 the resistance of the wire. I will, however, give one series of 

 experiments, in which I have compared a wire of mercury with 

 the cod of silver wire used in the previous experiments. The 

 comparison of a fluid with a solid metal was, I thought, emi- 

 nently calculated to test the accuracy of the law. 



A glass tube, 157 centimetres long and about 2'3 millimetres 

 in internal diameter, was fashioned into a spiral, as represented 

 in fig. 5. The tube was filled with mercury as high as the 

 bulbs aa. Connexion could be established between the pile and 

 the spiral by means of the copper wires bb, which dipped as far 

 as the centre of the bulbs aa. 



The coil of mercury, thus prepared, was immersed in 2 lbs. 

 11 oz. of water contained in a double -cased can, similar to the 

 one I have already described, and a current from a pile of five 

 cells was transmitted through it for ten minutes. The heat 

 evolved, the temperature of the room, and the deflections of the 

 galvanometer during the experiment, were carefully noted. Eight 

 of these experiments were made, in four of which the deflections 

 were on one side, and, in the other four, on the other side of the 

 meridian. 



Four experiments were made in a similar way with the coil of 

 sdver wire. In order to avoid the effects of any change in the 

 intensity of the earth's magnetism, these four experiments were 

 alternated with those made with the mercury coil. The thermo- 

 meter used in all the experiments was one of great accuracy ; 

 and each division of its scale corresponded to gjjj of a degree of 

 the Centigrade ;ale. 



