Mr. R. Phillips on the passive state of Iron. 511 



acquired an oily appearance. This acid was now placed in 

 the platinum crucible which was connected with one end of 

 the wire of the less sensitive galvanometer, an iron wire com- 

 municating with the other end ; the needle showed the passive 

 state was readily assumed by the iron and this mixture of per- 

 oxide of hydrogen and sulphuric acid. When the iron having 

 no connexion with the platinum was plunged into this mixture, 

 it produced gas ; but after having brought the iron in contact 

 with the crucible, the evolution of gas on the surface of the 

 iron ceased ; gas continued to be produced, as before contact, 

 on the platinum, which arose from its action on the oxy- 

 genized acid. The acid was now increased to about four times 

 its former bulk by the addition of water, and was found still 

 capable of assuming the passive condition. The current from 

 active iron, platinum and oxygenized sulphuric acid, produced 

 a much larger deflection of the needle than sulphuric acid, 

 either dilute or concentrated. The peroxide of barium was 

 prepared by Wohler and Liebig's process, and contained no 

 chloride, nitrate or chlorate. I found it advisable, when I 

 wished to observe the passive state with oxygenized sulphuric 

 acid, to be careful not to remove any portion of the iron from 

 the acid when once wetted by it ; and to avoid using the iron 

 when at all heated, as it is apt to become by being cleaned 

 with sand-paper. 



It was observed by Daniel, that a mixture of nitric and sul- 

 phuric acids does not act on iron ; and I have found that when 

 to a dilute sulphuric acid, one measure acid and six water, a 

 little bichromate of potash is added, the acid and iron can 

 easily be made mutually passive by contact with platinum ; so 

 that if a substance can exhibit the passive state, its mixture 

 with sulphuric acid does not annihilate this property. The- 

 nard found iron was without action on peroxide of hydrogen. 

 That this is really owing to the formation of the passive state 

 is, I think, shown by the action of oxygenized sulphuric acid, 

 in which I look upon the peroxide of hydrogen as acting 

 towards the iron as nitric or chromic acid would under similar 

 circumstances. 



There are now, then, the following bodies with which pas- 

 siveness has been developed, — nitric acid, peroxide of hy- 

 drogen, chromic acid, iodic acid and chloric acid. Peroxide 

 of hydrogen is an electrolyte, and was found so, I think, by 

 Thenard ; and solutions of nitric, chloric, iodic, and chromic 

 acids are electrolytes; all these substances are also easily de- 

 composed by giving up their oxygen, so that their molecules 

 have a tendency at an anode (but not at a cathode) to combine 

 with the iron entire, and also to decompose and so oxidate 



