Cast Steel. 
389 
heat ; polished steel is sooner tinged by heat, and with 
higher colours than iron. This is supposed to be a par- 
tial oxydation of the surface of the metal, but as it can 
take place under melted lead, or hot mercury (or hot oil) 
j not exposed to the air, this explanation is doubtful ; in a 
calcining heat, it suffers less by burning than iron, but 
by repeated heating and hammering with exposure to 
air, the charcoal or carbon can be gradually burnt away ; 
| the same effect takes place by gradual, long repeated ham- 
■ mering alone ; in a calcining heat, a light blue flame 
hovers over steel, probably owing to the gradually burn- 
ing of its carbon ; in a white heat exposed to the bellows, 
i it sends off more, and brighter, and lighter- coloured 
sparks than iron ; when covered with powdered charcoal 
pressed down close upon it, and exposed to a strong heat, it 
becomes overloaded with carbon, and acquires a slight coat 
resembling, and indeed consisting of black lead (Kish.) 
j when hammered, its specific gravity is somewhat greater 
than iron ; it leaves a residuum of black charcoal when 
dissolved in acids, which pure malleable iron does not. 
The sulphureous acid is better for the purpose than the 
sulphuric. 
| Highly carbonated cast iron differs from steel, in as 
much as the texture and quality of the metal is not uni- 
form. Cast iron is scarcely malleable ; it is much more 
1 brittle than steel ; it is but slightly hardened or softened 
by heating and cooling ; it is fusible in a close vessel at 
130 of Wedgewood’s pyrometer; the black residuum 
on solution in acids, is more abundant in cast iron than 
in steel ; cast iron contains uncombined carbon, and tin- 
mettalized iron ; it is more sonorous than steeh 
All these properties are owing to the superior quantity 
of carbon or charcoal, that is united to it, partly mixed and 
partly in combination. 
3 D 
