B 



206 Messrs, Stodart and Faraday on [March, 



on these steels, are the differences produced when they are hard 

 and when soft. Mr. Daniel, in his interesting paper on the 

 mechanical structure of iron, published in the Journal of Science, 

 has remarked, that pieces of hard and soft steel being placed in 

 muriatic acid, the first required five fold the time of the latter to 

 saturate the acid ; and that when its surface was examined, it 

 was covered with small cavities like worm-eaten wood, and was 

 compact and not at all striated, and that the latter presented a 

 fibrous and wavy texture. 



The properties of the platina alloy have enabled us to observe 

 other differences between hard and soft steel equally striking. 

 When two portions of the platina 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 examined, the hard 

 piece presents a covering of a metallic black carbonaceous pow- 

 der, 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 produced 

 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, exactly 

 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 combina- 

 tion with part of the metal and the charcoal, and forms an inflam- 

 mable 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 



