422 Dr. Tyndall on the Reduction of Temperatures 



very well, we will discard it, and proceed at once to an experi- 

 inentum crucis. If the arms A' B' are not actually included in 

 the voltaic circuit, they may seem to be in suspicious connexion 

 with it. We must remove this source of doubt. 



Experiment No. 3. — A and B, fig. 3, represent, as before, 

 the bismuth and antimony couple, united at one end. M is 

 a small chamber, hollowed out in a piece of cork and filled 

 with mercury. A' B' is a second delicate thermo-electric pair, 

 connected with the galvanometer, but wholly unconnected with 

 A B. The wires zv iv' are sufficiently strong to suppoit A' B', so 

 that the junction stands vertically over M, a slight pressure 

 being sufficient to cause the wedge-shaped end of the pair to 

 descend into the chamber of mercury. The whole arrangement 

 was permitted to remain in a room until the temperature of the 

 surrounding atmosphere was attained. Matters being in this 

 state, when the pair A' B', which I will call the test-pair, was 

 dipped into the mercury M, no effect was produced on the gal- 

 vanometer. Now the mercury must partake of the changes of 

 temperature of the junction with which it is in contact, and the 

 nature of these changes will be ascertained with great precision 

 by examining the mercury at proper intervals by means of the 

 test-pair. 



The voltaic circuit was closed, and the current allowed to 

 circulate for three minutes, passing in the first place from bis- 

 muth to antimony. The current was then interrupted, and the 

 test-pair was immediately dipped into the pool of mercury ; the 

 index of the galvanometer was driven through an arc of 



40^. 



The deflection was similar to that produced by immersing the 

 end of the test-pair in a freezing mixtm-e. Hence in this case 

 heat was undoubtedly abstracted from the mercury during the 

 passage of the current. 



The apparatus being permitted to resmne its equilibrium, the 

 voltaic current was caused to traverse AB in an opposite direc- 

 tion. At the end of three minutes the test-pair was again im- 

 mersed, and a deflection of 



45° 



was the consequence. The deflection was opposed to the former 

 one, and demonstrated the generation of heat at the junction. 



I am at present unable to see what possible objection can be 

 brought against this last experiment. A hygrometric effect is 

 out of the question ; and the test-pair A'B' being wholly uncon- 

 nected with the voltaic current, cannot in any way be influenced 

 by the latter. The results observed are evidently pure effects of 

 the heating and cooling of the junction. 



