The hardening of steel instruments is performed by heating them to a cherry-red, and then immersing them in cold water. The tempering is another process, calculated, as we observed, to obtain a fine and durable edge. This is performed by heating oil to a certain temperature, and plunging the instrument into it, where it remains until the colour appears, indicative of the particular kind of temper which is intended to be given. The experiments of Stoddart, (_Nicholson's Quarto Journal_, iv, 129,) are conclusive on this subject; for his experiments prove, that, between 430° and 450° the instrument assumes a pale yellowish tinge: at 460° the colour is a straw-yellow, and the instrument has the usual temper of pen-knives, razors, and other fine edge tools. The colour gradually deepens as the temperature rises, and at 500° becomes a bright brownish metallic yellow. As the heat increases, the surface is successively yellow, brown, red, and purple, to 580°, when it becomes of a uniform deep blue, like that of watch springs. Before the instrument becomes red-hot, the blue changes to a water colour, which is the last distinguishable colour. These different shades are owing to the oxidizement of the surface of the metal; and the art of ornamenting _sword-blades_, knives, &c. long practised in Sheffield, depends on this principle. The general process is, that an oily composition is used, with which flowers and various ornaments are painted. On the application of the heat required for tempering it, that part which was covered with the composition, is not altered, whereas, the uncovered parts of the blade are changed. These ornaments, when the paint is removed, have the natural colour of polished steel. When steel is heated in hydrogen gas, no appearance of the kind takes place, a fact which shows, that it is owing to the oxidizement of the metal. Iron is soluble in the acids. By the assistance of water, it is acted upon by sulphuric acid; the metal being oxidized, and the oxide dissolved, while hydrogen gas is evolved. The salt, formed in this case, is the sulphate of iron, green vitriol, or copperas. With muriatic, nitric, acetic and other acids, it forms various salts; and with gallic acid, when the iron is peroxidized, it forms the pergallate of iron, or common writing ink, and also the bases of black dye. Iron unites with carbon, sulphur and phosphorus. Of the sulphurets, there are two kinds, the protosulphuret and persulphuret. The former is the magnetic pyrites, and the latter, cubic pyrites, from both of which, green vitriol is obtained by decomposition. Pyrites, we may observe, was the original fire-stone, or the _feuer-stein_ of the Germans, which was used in the place of flint. See _Beckman's History of Invention_. Iron also unites with some of the metals, forming alloys. The white iron of the French, (_Fer blanc_,) or tin plate of the English, is found to be any alloy of tin with iron, as well as a covering of tin on iron.