Cast Steel. 
417 
formed in the piece and thus spoil it. Further, this con- 
traction is so well known, that when castings of particu- 
lar dimensions are required, the mould is c distantly made 
larger than the pattern by ci or sometimes to allow 
for the shrinking. Now it is impossible that this should 
happen if the specific gravity of the melted metal really 
exceeded that of the solid metal : how then does it han- 
A 
pen that the solid will float on the fluid metal ? the answer 
to this is not perhaps very obvious ; but it may be re- 
marked that not only solid cast iron but even bar iron 
which is of considerably greater specific gravity, and not 
only bar iron but even lead which is more than half as 
heavy again as cast iron, will float upon its surface. But 
though cast iron like all other metals shrinks when it be- 
comes solid, yet at the instant of congealing it appears to 
undergo a momentary expansion, and thus takes a remark- 
ably perfect impression of any pattern with which it comes 
in contact. 
Bar iron is of a bluish white colour, has a' fibrous hack- 
ly fracture, is malleable both when hot and cold, and is 
capable of uniting with another piece of bar iron by weld- 
ing: it may be drawn into very fine wire and is the most 
tenacious of all metals, a wire of an inch in diameter 
being capable of sustaining from 450 to SQOihs. before it 
breaks. It is fusible, but requires for this purpose a high- 
er heat than cast iron. Its specific gravity is subject to 
some variations : that of common hammered iron, accord- 
ing to Dr. Pearson, is from 7.45 to 7.6 : Swedish bar 
iron varies between 7.70 and 7.78. It expands like 
all other metals by heat : the amount of its expansion for 
every degree of Fahrenheit’s thermometer between the free- 
zing and bailing point of water, is equal to 0.000006358. 
Steel is of a light-grey colour and a fracture more or 
less fine granular : it is harder and more brittle than bar 
iron in proportion to the quantity of carbon that it com 
