418 



TENTH ANNUAL REPORT OF 



or, in other words, under such circumstances it becomes alternately 

 active and passive, and this haj^pens at first at intervals of about one 

 second, but during the course of the action the intervals become 

 shorter until finally rapid action commences. 



Let each of the conducting wires of a powerful sim- 

 ple battery C, Fig. 46, pass into a small cup filled 

 with mercury, and connect the cup a in which the neg- 

 ative wire dips by a platinum strip p, with the liquid 

 (11 parts by vol. of water to 1 part of sulphuric acid) 

 of the decomposing cell g ; then dip one end of an 

 ordinary iron wire e in the positive mercury cup &, and 

 the other end in the acidified water in the decomposing 

 cell ; the iron will become passive, and no hydrogen 

 will be developed at the platinum electrode 73, since, on 

 account of the polarization at^, the electro-motive force 

 of will not suffice to send a sensible current through 



But if the battery be closed in another way, for instance, so that 

 the iron wire e may first dip in g and then in h, it will not become 

 passive ; g itself becomes an exciting cell^ whose current combines with 

 that of the constant elements, and thus a lively development of hy- 

 drogen will appear atp, during which the iron wire is dissolved. 



If the battery be so closed that e is passive, and that consequently 

 no hydrogen rises at p, various expedients may be adopted to make 

 e again active, so that the gas may begin to appear at p. One of the 

 means of producing this development consists in interrupting the 

 circuit at any point, and after a short time closing it again ; for ex- 

 ample, by drawing out the wire d from a, e at once becomes active, 

 and if d be now immersed again, a lively development takes place at 

 the platinum stripy. 



To obtain the passivity of e, the constant element must tend to 

 drive the current though g with a certain energy, on which account 

 it will cease when the circuit is interrupted. The energy with which 

 the constant element tends to drive the current through g, can be 

 weakened by introducing a good lateral circuit. 



If the mercury cups a and h be connected by a short thick copper 

 wire, nearly the whole current which the constant element is able to 

 generate, will pass through it ; e loses its passivity, and part of the 

 current generated by C passes through g, and exhibits itself by a devel- 

 oj^ment of gas. 



On the contrary, if while e is yet passive, a and 6 be connected by 

 a wire, which exerts considerable resistance, the current which it can 

 conduct is too feeble to overcome the passivity of the iron wire e ; by 

 such a wire no development of gas at ;> can be produced. 



Between these two limits of conductive capacity of the wires con- 

 necting the mercury cups a and b — namely, the very good conducting 

 wire, through which the passivity of e is totally destroyed, and a con- 

 tinuous development of gas atp is produced, and the very bad con- 

 ducting wire which cannot destroy the passivity of e — there is a certain 

 intermediate length of wire, by means of which the passivity of e is 



