Causes of dinmiished Activity of the Battery, 411 



altogether for a while, the apparatus resumed fts first power, 

 requiring, however, from five to ten minutes for this purpose; 

 and then, as before, on making contact between 3 and p, there 

 was again a momentary current, and immediately all the ef- 

 fects apparently ceased. 



1037. This effect I was ultimately able to refer to the state of 

 the film of fluid in contact with the zinc plate in cell i. The acid 

 of that film is instantly neutralized by the oxide formed ; the 

 oxidation of the zinc cannot, of course, go on with the same 

 facility as before; and the chemical action being thus inter- 

 rupted, the voltaic action diminishes with it. The time of 

 the rest was required for the diffusion of the liquid, and its re- 

 placement by other acid. From the serious influence of this 

 cause in experiments with single pairs of plates of different 

 metals, in which I was at one time engaged, and the extreme 

 care required to avoid it, I cannot help feeling a strong sus- 

 picion that it interferes more frequently and extensively than 

 experimenters are aware of, and therefore direct their atten- 

 tion to it. 



1038. In considering the effect in delicate experiments of 

 this source of irregularity of action in the voltaic apparatus, 

 it must be remembered that it is only that very small portion 

 of matter which is directly in contact with the oxidizable 

 metal which has to be considered with reference to the change 

 of its nature; and this portion is not very readily displaced 

 from its position upon the surface of the metal (582. 605.), 

 especially if that metal be rough and irregular. In illustra- 

 tion of this effect, I will quote a remarkable experiment. A 

 burnished platina plate (569.) was put into hot strong sul- 

 phuric acid for an instant only: it was then put into distilled 

 water, moved about in it, taken out, and wiped dry: it was 

 put into a second portion of distilled water, moved about in 

 it, and again wiped : it was put into a third portion of distilled 

 water, in which it was moved about for nearly eight seconds ; 

 it was then, without wiping, put into a fourth portion of di- 

 stilled water, where it was allowed to remain five minutes. 

 The two latter portions of water were then tested for sul- 

 phuric acid ; the third gave no sensible appearance of that 

 substance, but the fourth gave indications which were not 

 merely evident, but abundant, for the circumstances under 

 which it had been introduced. The result sufficiently shows 

 with what difficulty that portion of the substance which is in 

 contact with the metal leaves it; and as the contact of the 

 fluid formed against the plate in the voltaic circuit must be as 

 intimate and as perfect as possible, it is easy to see how 

 quickly and greatly it must vary from the general fluid in the 



3 G2 



