332 Sir J. F. W. Herschel on the. Voltaic Condition of Iron. 



acid diluted to the same, or even to a greater extent; which 

 proves that these phenomena are owing, not merely to the 

 absence of the water necessary to hold the nitrate of iron in 

 solution, but rather to a certain permanent electrical state of 

 the surface of the metal. This mode of considering the 

 subject is confirmed by the following experiments. 



A piece of iron wire was heated, and a small zone of wax 

 placed around the middle of it to divide it into two pro- 

 tions. The wire being immersed in the concentrated acid, 

 the action ceased at the same moment upon each half, and 

 upon touching one of its extremities with copper the action 

 was renewed in each simultaneously. The prepared state 

 being again established, the iron was withdrawn by a glass rod 

 attached to the wax, and one of its extremities was touched 

 while it was in the air. The action commenced as usual at 

 the extremity touched, and was extended through half of the 

 wire, but was then stopped by the wax, so that one half was 

 brown, while the other retained its metallic lustre. 



A piece of iron, bent into an arc and divided as I have de- 

 scribed by wax, was prepared, and then two- thirds of its length 

 withdrawn from the acid, thus leaving the greater part of one 

 half of it, A, still immersed. The other half (B) while thus 

 in the air was touched with copper, when the action was pro- 

 pagated to the wax, where it stopped. The extremity B was 

 then quickly lowered until it touched the surface of the acid ; 

 the action commenced immediately in the portion A, which 

 was immersed, and which had hitherto preserved its lustre. 



Prepared iron resists the action of the acid, when at a tem- 

 perature insupportable to the hand, but not at the tempera- 

 ture of ebullition. When it is let fall into very hot acid, it 

 resists for a few instants, and then begins to cause a violent 

 effervescence. I have never found that iron could be 

 submitted to the action of boiling nitric acid without oxida- 

 tion, as is remarked by M. Braconnot, but perhaps the acid 

 which he employed was more concentrated than mine. On 

 the other hand I have found it impossible to make acid, of the 

 density of 1*399, either cold or at the temperature of ebulli- 

 tion, act upon softened steel (acier recnit), or even upon those 

 plates of steel which are employed for watch-springs. It may 

 be kept boiling upon the plates for any length of time, without 

 producing the least effect. But a circumstance, which to me 

 appears very singular, is that a wholly different effect is pro- 

 duced upon steel which has received the highest temper, so 

 as completely to resist the file, it being attacked with extreme 

 violence by the hot acid, and even with considerable facility 

 by cold acid. But when the acid is cold the steel is easily 



