_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 several 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 extreme 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 restored, 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 different 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 cementation, with charcoal powder, in a peculiar kind of furnace. The iron produced in Sweden is considered superior to that of any other country in Europe for the manufacture of steel. All kinds of edge tools, where excellence is required, are made of steel; and a steel instrument may be immediately known from an iron one, by letting fall upon it a drop of nitric acid or aqua fortis (206), somewhat diluted with water. If it be steel, this will occasion a black spot; but if it be iron, it will not have this effect. Steel is attracted by the magnet, and is capable of receiving a permanent _magnetic property_, which has led to the discovery of the mariner’s compass. Had iron been productive of no other advantages to mankind than this, it would on this account alone have been entitled to their greatest attention.