of the Heat of Combustion. 103 



this way was associated in dilute sulphuric acid with a piece 

 of iron, the needle would deviate 63^° for some time, and then 

 gradually decline until it took up a permanent situation at 

 29\°. By experimenting in the same way with amalgamated 

 zinc as a positive element, I had a transitory deflection of 76°, 

 and a permanent deflection of 63°. 



22. Very trifling transitory effects were obtained by the im- 

 mersion of iron, when that metal was associated with amalga- 

 mated zinc. But this might have been anticipated, because 

 the transitory current is owing to the presence of oxygen on 

 the negative plate, and it ii obvious that the hydrogen evolved 

 by the local action of the iron, would, whilst in a nascent state, 

 combine with that oxygen, and thus prevent a great part of it 

 from exercising any influence upon the intensity of the current. 



23. An experiment was also made with an arrangement of 

 copper, amalgamated zinc, and dilute sulphuric acid. It was 

 able to deflect the needle 51° pretty permanently. On wash- 

 ing and drying the copper, and experimenting as in (16.), I 

 observed a transitory deflection of 72°. This experiment de- 

 serves attention, because it shows that the transitory current 

 occasioned by the copper is the same as that exhibited by pla- 

 tinized silver when experimented with in the same way (20.). 

 I take it as an argument, that when copper is in its best state 

 it forms with amalgamated zinc a battery as intense as the pla- 

 tinized silver. 



24<. That the transitory currents which we are discussing 

 are not occasioned by the diff"usion of the salt formed about 

 the positive element during the cessation of voltaic action, is 

 obvious from the fact that (when the jn-oper precautions are 

 observed) they are not produced by the agitation (8.), or by 

 the immersion (19. and 20.) of the metal about which the salt 

 is formed. And if anything can render this more evident, it 

 is the fact that the immersion of the copper plate of a Daniell's 

 battery causes the needle to advance little or no higher than 

 its permanent situation, as might have been anticipated from 

 the theory which refers the transitory effects to chemical com- 

 bination at the negative plate, on account of the slight affinity 

 of copper for oxygen. The following experiments are also de- 

 cisive of this question. 



25. A glass jar, «, fig. 1, containing some dilute sulphuric 

 acid, was placed upon the plate 2^ P <'f ^'i air-pump. A small 

 rod of iron, z, was immersed in the liquid, and connected by 

 means of the pump-})iate to the galvanometer (5.). An open 

 receiver, r, was now placed over the jar, and the ground brass 

 plate b, with its sLufling-box and sliding rod (the latter having 

 the small piece of platinized silver, s, affixed to its extremity), 



