Cast Steel. 
401 
' 
| with a file or with emery paper, is to be spread over with 
very dilute nitrous acid, by which the iron will be dissolv- 
| ed, but the carbon will remain behind untouched ; after 
therefore the acid has been allowed to act for a few minutes, 
the bar is to be put into clean water and moved about in 
1 it gently, that both the residual acid and the nitrat of iron 
may be washed away, care being taken not to touch the 
| surface with the hand or any thing else that may rub off 
i the carbon. The bar thus washed, if pure iron, will exhi- 
bit an uniform iron-grey colour ; if it be pure steel, the 
colour of the surface will be black, the iron having been 
taken up by the acid and a thin coating of carbon remain- 
! ing ; but if it be a mixture of iron and steel, the surface 
will be dotted or streaked, those parts which are steel be- 
ing of a dull black, and those which are iron exhibiting the 
Usual colour and lustre of this metal. 
Steel being considerably more expensive than iron, it 
is customary in making the larger and coarser kinds of 
cutting instruments to form only the edge of steel The 
two bars of iron and steel are first welded together and 
afterwards forged into the requisite shape in the usual 
manner. Highly carbonized steel is however incapable 
of being thus united to iron, because the same tempera- 
ture at which iron welds freely, is that, at which this kind 
of steel enters into fusion, and therefore the first stroke of 
the hammer will entirely shatter the steel and disperse it 
about in small fragments. This however is a difficulty 
which it is well worth while taking some pains to over- 
come, as the efficacy and durability of instruments thus 
composed materially, depends upon the goodness of the 
steel. The most effectual way hitherto discovered of 
uniting together iron and highly carbonized steel, is that 
published by Sir Thos. Frank land. The iron is to be 
raised to a welding heat, in one forge, and the steel is to 
,be made as hot as it can bear without becoming very brit- 
