STEEL.
It is probably owing to a total deprivation of vital air which it holds with so great avidity, that iron on being kept many hours or days in ignited charcoal becomes converted into steel, and thence acquires the faculty of being welded when red hot long before it melts, and also the power of becoming hard when immersed in cold water; both which I suppose depend on the same cause, that is, on its being a worse conductor of heat than other metals; and hence the surface both acquires heat much sooner, and loses it much sooner, than the internal parts of it, in this circumstance resembling glass.
When steel is made very hot, and suddenly immerged in very cold water, and moved about in it, the surface of the steel becomes cooled first, and thus producing a kind of case or arch over the internal part, prevents that internal part from contracting quite so much as it otherwise would do, whence it becomes brittler and harder, like the glass-drops called Prince Rupert's drops, which are made by dropping melted glass into cold water. This idea is countenanced by the circumstance that hardened steel is specifically lighter than steel which is more gradually cooled. (Nicholson's Chemistry, p. 313.) Why the brittleness and hardness of steel or glass should keep pace or be companions to each other may be difficult to conceive.
When a steel spring is forcibly bent till it break, it requires less power to bend it through the first inch than the second, and less through the second than the third; the same I suppose to happen if a wire be distended till it break by hanging weights to it; this shews that the particles may be forced from each other to a small distance by less power, than is necessary to make them recede to a greater distance; in this circumstance perhaps the attraction of cohesion differs from that of gravitation, which exerts its power inversely as the squares of the distance. Hence it appears that if the innermost particles of a steel bar, by cooling the external surface first, are kept from approaching each other so nearly as they otherwise would do, that they become in the situation of the particles on the convex side of a bent spring, and can not be forced further from each other except by a greater power than would have been necessary to have made them recede thus far. And secondly, that if they be forced a little further from each other they separate; this may be exemplified by laying two magnetic needles parallel to each other, the contrary poles together, then drawing them longitudinally from each other, they will slide with small force till they begin to separate, and will then require a stronger force to really separate them. Hence it appears, that hardness and brittleness depend on the same circumstance, that the particles are removed to a greater distance from each other and thus resist any power more forcibly which is applied to displace them further, this constitutes hardness. And secondly, if they are displaced by such applied force they immediately separate, and this constitutes brittleness.
Steel may be thus rendered too brittle for many purposes, on which account artists have means of softening it again, by exposing it to certain degrees of heat, for the construction of different kinds of tools, which is called tempering it. Some artists plunge large tools in very cold water as soon as they are compleatly ignited, and moving it about, take it out as soon as it ceases to be luminous beneath the water; it is then rubbed quickly with a file or on sand to clean the surface, the heat which the metal still retains soon begins to produce a succession of colours; if a hard temper be required, the piece is dipped again and stirred about in cold water as soon as the yellow tinge appears, if it be cooled when the purple tinge appears it becomes fit for gravers' tools used in working upon metals; if cooled while blue it is proper for springs. Nicholson's Chemistry, p. 313. Keir's Chemical Dictionary.