Mr Stromeyer on Metallic Iron and its Oxides. 301 



bo far as regards the combustibility of iron. The Professor 

 remarks, that, in order to obtain iron with ease and certainty 

 in a perfectly metallic state, by means of hydrogen, it is ne- 

 cessary to conduct the gas, previously dried by the chloride of 

 calcium, over the peroxide of iron at a red heat. The process 

 does indeed succeed at temperatures which are much below a 

 red heat ; but the reduction in these instances takes place very 

 slowly, so that it is exceedingly difficult in this way to prepare 

 metallic iron perfectly free from the protoxide. 



Professor Stromeyer maintains that pure metallic iron ob- 

 tained by the preceding process, however low the temperature 

 which may have been employed in its reduction, does not pos- 

 sess the property of burning spontaneously ; but that on being 

 heated to the degree at which cadmium fuses, it then sud- 

 denly takes fire, and burns with emission of heat and light till 

 the whole of it is converted into the black oxide. But if hy- 

 drogen gas is conducted over the red oxide of iron at a tem- 

 perature still lower than that at. which complete reduction is 

 effected, a partial deoxidation ensues, and the peroxide is con- 

 verted into the real protoxide of iron. Professor Stromeyer 

 employs the term real protoxide, because this oxide, previous 

 to his experiments, has never been obtained in an insulated 

 state, and because the black oxide, procured by passing watery 

 vapour over metallic iron, though commonly mistaken for the 

 protoxide, is in reality a compound of the protoxide and pe- 

 roxide of iron. 



The real protoxide of iron has a dark blackish blue colour, 

 which appears almost black by reflected light. It stains glass 

 blue, and is the cause of the blue colour of iron slag. This 

 protoxide is combustible in a high degree. If, after its forma- 

 tion, it is completely protected from the atmosphere by being 

 kept in hydrogen gas till quite cold, it will take fire the instant 

 it is placed in a saucer, so as to be completely exposed to the air; 

 but instead of passing, like metallic iron, into the black oxide, 

 it is converted at once, and completely, into the peroxide. 



Professor Stromeyer ascribes the spontaneous combustion 

 of the metallic iron in the experiment of M. Magnus to this 

 protoxide, the presence of which it is difficult to avoid alto- 

 gether, when the peroxide is reduced by hydrogen at a low 



