which admirably answers the purpose. Other means of protection, such as plunging the heated steel in oil, hot or cold, or in melted lead, or a composition of metals, are uncertain in their results, and liable to failure; because, even if the oil, metals and heat are always the same, the steel is notone kind of steel, or a particular kind of work, acquiring more hardness by the same treatment than another.

HARDENING BY COMPRESSION.

Among the various methods of hardening is that in spring-water, the most simple and most safe; but there are some small articles to which we cannot give their highest degree of hardness and tenacity in this way. These are engravers' tools, surgical instruments, &c., which may be hardened to a high degree by being hammered with a very small hammer, well polished, on a hard, polished anvil. Delicate instruments assume by this practice a high degree of hardness, and a finer edge and more elasticity than can be given to them by any other mode of hardening. The conical holes in the wire draw-plate are hardened in the same way.

ANNEALING.

Of steel is a necessary operation in all cases where filing or engraving is to be done. The steel, as it comes from the anvil, is too hard for the file and the chisel, and must be softened or annealed before it is ready for the engraver. The common method of annealing is to heat the steel to a gentle redness off the tuyere, and leave it in the ashes of the hearth until cold. The slower this operation is performed, the more uniform and soft will the steel be. Tempering in a pot, imbedded in sand or chalk, or any dry powder, is preferable to the open fire. Some authorities recommend pastes and powders of various compositions for annealing; but all such preparations are fallacious. Nothing more is requisite than heat, and the exclusion of atmospheric air or oxygen.

TEMPERING.

Steel properly hardened, is as hard as its peculiar quality permits it to become. In this state it is generally too brittle to be of any practical use, and it is necessary to temper it before it is exposed to any strain on its tenacity. Small tools are generally

tempered after hardening, by covering the surface with a film of tallow or oil, then heating the steel until the oil diffuses a black smoke, or burns, or ceases to burn, and then plunging it in cold water. Picks, mattocks, blasting tools, and similar implements, are tempered by heating the heavy part from behind the edge or point, driving the heat towards the point. One side of the edge being ground white, shows the tempering colours; and when the proper colour is arrived at, the steel is cooled just at the point, but not the heavy iron behind it. Many mechanics harden and temper their common tools in the same heat, by merely dipping the hot point or edge in cold water; the heat of the heavier parts is then transmitted to the hardened edge, after it is removed from the cold water. When the proper colour is gained, which is ascertained by scratches of a dull file, the tool is cooled by dipping it in water. This latter process requires some experience, or the steel is apt to become either too hard or too soft, and require renewed hardening; which, of course, is injurious to the steel.

Instruments which are designed to be very perfect, are polished all over, and then heated to the tempering colour. Small articles, such as knife-blades, are set in large numbers with their tangs in a heavy steel

or iron plate; that plate is then heated, and, when the proper colour is on the blades, each is singly plunged into cold water. Needles are tempered in masses, by burning oil upon them. Saw-blades, and large articles generally, are tempered in hot sand; the sand being heated to a certain point, which is tested by the thermometer. Sometimes this precaution is not taken; and the course then is to watch the articles until they obtain the requisite colour, when they are hardened in either air or water.

The colours to which steel can be tempered may be approximately stated thus: The hardest articles, which do not require much strength, should assume a faint yellow; surgical instruments, razors, and engravers' tools, a pale straw-colour; knives, cold chisels, and bore-bits, yellow; chisels, shears, hammers, anvils, and some varieties of saw-blades, dark yellow; axes, plane-irons, carpenters' tools generally, and most edged tools, brownish purple; table-knives, weapons, and scissors, purple; watch-springs, saws, and augers, light blue; common saws, heavy watchsprings, carriage-springs, and springs generally, blue; articles which require strength, but in which hardness is a secondary consideration, dark blue. Beyond dark blue the colour is black, and the steel is perfectly soft.

These colours are only approximating the subject; for the various kinds of steel will show a different degree of hardness in being tempered to the same colour. The naturally soft steel should have a shade or two less temper than that of the hardest description.

Many propositions have been made by scientific men to harden steel in fusible metal compositions, to avoid tempering; or to temper the steel in such metals; or to temper in a bath of lead heated to a certain degree, measured by the thermometer, &c. All these things are very well as scientific recommendations, and we shall speak of them in another place. They are of little practical value, however; for it is not the absolute degree of heat in hardening, or the difference in heat and cold, or the degree of the tempering bath, which decides the superiority or inferiority of hardened instruments of steel. The quality and description of the steel, the manner and mode of working it, the form and the fuel, are matters which influence the degree of heat in hardening, and also in tempering. In all cases of this kind, the simplest way of working is the best; the skill and dexterity of the worker in steel is a better guarantee of success than all the artificial compositions of cooling and tempering mediums. A good, skilful work