192 IRON. 
greater than even that of gold. By means of this me- 
tal the earth has been cultivated and subdued. With- 
out it houses, cities, and ships, could not have been 
built ; and few arts could have been practised. It forms 
also the machinery by which the most useful and impor- 
tant mechanical powers are generated and applied. 
Steel is usually made by a process called cementation. 
This consists in keeping bars of iron in contact with 
powdered charcoal, during a state of ignition, for seve- 
ral hours, in earthen troughs, or crucibles, the mouths 
of which are stopped up with clay. Steel, if heated to 
redness, and suffered to cool slowly, becomes soft ; but 
if plunged, whilst hot, into cold water, it acquires ex- 
treme hardness. It may be rendered so hard as even. 
to scratch glass ; and at the same time, it becomes 
more brittle and elastic than it was before. Although 
thus hardened, it may have its softness and ductility re- 
stored, by being again heated, and suffered to cool 
slowly. A piece of polished steel, in heating, assumes 
first a straw-yellow colour, then a lighter yellow, next 
becomes purple, then violet, then red, next deep blue, 
and at last of all bright blue. At this period it becomes 
red hot, the colours disappear, and metallic scales are 
formed upon, and encrust its surface. All these differ- 
ent shades of colour indicate the different tempers that 
the steel acquires by the increase of heat, from that 
which renders it proper for files, to that which fits it for 
the manufacture of watch springs. Mr. Stoddart has 
availed himself of this property to give to surgical, and 
other cutting instruments, those degrees of temper 
which their various uses require. 
The kind of steel which has been most celebrated in 
this country is that imported from Syria under the name 
of Damascus steel. Germany is also noted for its steel. 
The best steel manufactured in Britain is known by the 
name of cast steel ; and the making of it, although it 
was long kept a profound secret, is now discovered to 
be a simple process. It consists merely in fusing it 
with carbonat of lime (140), or in what is called cemen- 
