1 822.] On the Alloys of Steel 77 



observe other differences between hard and soft steel equally 

 striking. When two portions of the platinum alloy, one hard 

 and one soft, are put into the same diluted sulphuric acid and 

 suffered to remain for a few hours, then taken out and ex- 

 amined, the hard piece presents a covering of a metallic black 

 carbonaceous powder, and the surface is generally slightly 

 fibrous ; but the soft piece, on examination, is found to be 

 covered with a thick coat of grey metallic plumbaginous matter, 

 soft to the touch, and which may be cut with a knife, and its 

 quantity seven or eight times that of the powder on the hard 

 piece : it does not appear as if it contained any free charcoal, 

 but considerably resembles the plumbaginous powder Mr. 

 Daniel describes as obtained by the action of acid on cast iron. 



The same difference is observed if pure steel be used, but 

 it is not so striking ; because, being much less rapidly attacked 

 by the acid, it has to remain longer in it, and the powder pro- 

 duced is still further acted on. 



The powder procured from the soft steel or alloy in these 

 experiments, when it has not remained long in the acid, ex- 

 actly resembles finely divided plumbago, and appears to be a 

 carburet of iron, and probably of the alloying metal also. It 

 is not acted on by water, but in the air the iron oxidates and 

 discolours the substance. When it remains long in the acid, 

 or is boiled in it, it is reduced to the same state as the powder 

 from the hard steel or alloy. 



When any of these residua are boiled in diluted sulphuric 

 or muriatic acid, protoxide of iron is dissolved, and a black 

 powder remains unalterable by the further action of the acid ; 

 it is apparently in greater quantity from the alloys than from 

 pure steel, and when washed, dried, and heated to 300 or 400 

 in the air, burns like pyrophorus, with much fume: or if lighted, 

 burns like bitumen, and with a bright flame ; the residuum is 

 protoxide of iron, and the alloying metal. Hence, during the 

 action of the acid on the steel, a portion of hydrogen enters 

 into combination with part of the metal and the charcoal, and 

 forms an inflammable compound not acted upon by the acid. 



Some striking effects are produced by the action of nitric 

 acid on these powders. If that from pure steel be taken, it is 

 entirely dissolved ; and such is also the case if the powder be 

 taken from an alloy the metal of which is soluble in nitric acid ; 



