The good quality of steel is shown by its being homogeneous, being easily worked at the forge, by its hardening and tempering well, by its resisting or overcoming forces, and by its elasticity. To ascertain the first point, the surface should be ground and polished on the wheel, when its lustre and texture will appear. The second test requires the giving it a heat suitable to its nature and state of conversion. The size and color of the grain are best shown by taking a bar forged into a razor form, hardening and tempering it, and then breaking off the thin edge in successive bits with a hammer and anvil. If it had been fully ignited only at the end, then, after the hardening, it will display, on fracture, a dissimilarity in the aspect of itsgrains from that extremity to the other, as they are whiter and larger at the former than the latter. The other qualities become manifest on filing the steel, using it as a chisel for cutting iron, or bending it under a heavy weight. Kinman long ago defined steel to be any kind of iron which, when heated to redness, and then plunged into cold water, becomes harder. But several kinds of cast iron are susceptible of such hardening. Every malleable and flexible iron, however, which may be hardened in that way is steel. Moreover, steel may be distinguished from pure iron by its giving a dark gray spot when a drop of dilute nitric acid is let fall on its surface, while iron affords a green one. Exposed to the air, steel rusts less rapidly than iron.