512 Mr. R. Phillips on the passive state of Iron. 



the iron. I think from this, that the passive state results from 

 these two forces acting simultaneously and balancing each 

 other. A parallel case is perhaps presented by hydrocyanic 

 acid, which readily decomposes when pure with the formation 

 of a brown substance and ammonia ; and by concentrated 

 mineral acids it is rapidly converted, together with the elements 

 of water, into formic acid and ammonia, while with a certain 

 small quantity oi a strong mineral acid neither of these two 

 modes of decomposition appear. 



In these experiments with nitric, chloric, iodic and chromic 

 acids, it is seen that the passive state is not absolute; the 

 mixture of peroxide of hydrogen and sulphuric acid was not 

 examined in this particular, but I cannot doubt it forms no 

 exception. The current between passive iron and platinum 

 was always the same in direction as between zinc and platinum 

 in sulphuric acid. When two clean iron wires were immersed 

 in a mixture of about equal volumes of nitric acid, specific 

 gravity 1*39, and sulphuric acid, the galvanometer always 

 showed that one of the wires could act to the other as zinc to 

 platinum, the current being in different experiments various 

 both in force and direction. 



The following experiment shows, independently of the gal- 

 vanometer, the existence of chemical action during the passive 

 condition. I took a piece of clean iron wire 2 l n th of an inch 

 thick, and about 1^ inch long, and twisted some platinum 

 wire about one end of it, leaving a portion of the platinum 

 wire projecting ; then laying hold of this projecting portion, 

 the iron was placed in half a fluid ounce of nitric acid, specific 

 gravity 1*39, and left there for about two hours; the iron was 

 then removed by means of the platinum to another half-ounce 

 of the same acid, in which it was well-stirred about to wa*li 

 it, and then the metals were thrown into a vial containing a 

 similar quantity of the same acid ; the whole of the iron lay 

 beneath the surface of the acid, and the vial was closed and 

 put away in a dark place. Examined at the end of two days, 

 the acid was found to be rather coloured ; it was poured off 

 the iron, evaporated nearly to dryness, and a solution of am- 

 monia added, which caused an abundant precipitate of the ses- 

 quioxide of iron. 



The comparative perfection of the passive state with chromic 

 acid, even when very dilute, suggests that bichromate of 

 potash would be a useful addition to the nitro-sulphuric acid 

 employed in Schbnbein's voltaic pile. 



7 Prospect Place, Ball's Pond Road, 

 November 23, 1848. 



