THE SMITHSONIAN INSTITUTION. 417 



consideration, Sclionbein decided upon the following form for the ex- 

 periment. One of the ends ot a long iron wire was coated with hyper- 

 oxide of lead, and the end P thus prepared was immersed in a solution 

 of sulphate of copper ; the wire was then bent, and the unprepared 

 end E was also immersed in the liquid. E indicated passivity ; no 

 copper was precipitated. 



While E was becoming passive, the hyper-oxide of lead gradually 

 disappeared at P, and P became active as soon as the hyper-oxide 

 which covered this end had totally disappeared. 



In the transfer of passivity from one iron wire to 

 another in nitric acid, represented in Fig. 45, the 

 protecting film of oxide on E is evidently produced 

 by the necessary quantity of oxygen being immedi- 

 ately brought by the current to the end E of the 

 wire. But the current, which liberates oxygen at 

 E, must develop hydrogen at P, which attracts the 

 oxygen from the protecting oxide film of P ; thus 

 one would think that the same current which occa- 

 sions the formation of the protecting film around E, 

 must also occasion its removal from P ; or^ in other 

 words, that rendering E passive would make P 

 active, provided that P itself is only a secondary 

 passive wire, and consequently not protected by a 

 very thick film. 



But the experiment shows, that with a secondary passive wire, in 

 nitric acid of 1.36 specific gravity, another can be made passive with- 

 out the first becoming active, which is probably owing to the fact that 

 the hydrogen set free is, at least in part, oxidized by the nitric acid, 

 and thus the film of oxide cannot be wholly reduced. But if the cur- 

 rent should continue longer, as is the case when instead of E a zinc 

 or copper strip be let down into the acid at P, neither of which be- 

 comes passive, the protecting film will be immediately dissolved from 

 P, and P itself will become active. 



P can be rendered active again, even with an iron wire, if dilute 

 acid be used. 



§51. Pulsations of 2^cissivity. — With reference to the energy with 

 which the nitric acid attacks an iron wire, there are two principal de- 

 grees to be distinguished, which we shall call the sloiv and the rapid 

 action. The slow action is characterized by ceasing, instantly, as 

 soon as the iron wire is touched by a platinum wire immersed in the 

 acid ; the iron thus exposed to the slow action of the acid became pas- 

 sive in this way. On an iron wire which is exposed to the rapid ac- 

 tion of the acid, and on which a lively development of gas takes place, 

 this treatment with a platinum wire has no influence ; it does not be- 

 come passive by such means. 



If an iron wire, ^rendered passive by repeated immersions in nitric 

 acid of the spec. grav. 1.35, be touched, while yet in the liquid, with 

 a- copper or brass wire, which is at the same time dipped into the acid, 

 the iron wire, as already shown, becomes active, and is subjected to 

 slow action. This activity^ however, is not constant, but intermittent ; 

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