crucible, which has another aperture to allow that part of the gas to escape, which has not been taken up by the metal. Steel, in ingots, is porous; but, to confer solidity, it is hammered, tilted, and rolled. At Attercliffe, near Sheffield, are extensive works for these purposes. Here, by the power of a water-wheel, fifteen feet in diameter, hammers are worked, weighing from 3 to 4 cwt. and strike, at ten or twelve inches fall, from 100 to 220 times in a minute. The ingots, at a strong red heat, are exposed to the action of these hammers, and the metals condensed into bars, which are next submitted, at the same degree of heat, to the tilting hammer, which gives 300 strokes per minute : lastly, they are rolled or flattened into sheets, and drawn into lengths. Six tons a week are hammered down by one hammer; about three tons are tilted; and twenty-four tons can be rolled, working night and day, by relays of hands.

The making of steel is a British manufacture scarcely sixty years old. Previously it came from Austria and Styria, and was dear and little used. It is, however, now heated, welded, cut, and moulded in this country, with nearly the same facility as deal wood by an ordinary carpenter.

Why does a drop of nitric acid, let fall upon steel, occasion a black spot?

Because the iron is dissolved, and the carbon thereby exposed to view.—Parkes.

Why is steel tempered?

Because, when steel is heated to a cherry-red colour, and then plunged into cold water, it becomes so extremely hard and brittle, as to be unfit for almost any practical purpose; and tempering reduces it from this extreme hardness, by heating it to a certain point or temperature.

The polishing of steel is not executed in the same manner as that of the softer metals: the steel is not

polished until it has been hardened, and the harder it is, the more brilliant will be its polish. Rotten-stone, a kind of very light tripoli, but finer than the other sorts, and found near Bakewell, in Derbyshire, is esteemed for general polishing; but steel, from its extreme hardness, requires to be polished with emery.

Why are various colours produced on heated steel? Because of the oxidation which takes place, as is proved from the circumstance that when steel is heated and suffered to cool under mercury or oil, none of the colours appear; nor do they when it is heated in bydrogen or nitrogen.—Brande.

Why is it customary to judge of the temper of steel by its colours?

Because, the surface being a little brightened, exhibits, when heated, various colours, which constantly change as the temperature increases. Thus, when steel is placed in a bath heated to 600o, the first change is at about 430°, which is very faint; at 460o, the colour is straw, becoming deeper as the temperature is increased; at 500°, the colour is brown; this is followed by a red tinge, with streaks of purple, then purple; and at nearly 600°, it is blue. The degrees at which the different colours are produced, being thus known, the workman has only to heat the bath with its contents up to the required point. For example, suppose the blade of a pen-knife, (or a hundred of them,) to require tempering; they are suffered to remain in the bath until the mercury in the thermometer rises to 460o, and no longer, that being the heat at which the knife (supposing it to be made of the best English cast steel) will be sufficiently tempered.

Why is cast steel so called?

Because it is prepared by fusing blistered steel with a flux composed of carbonaceous and vitrifiable ingredients, casting it into ingots, and afterwards by gentle heating, and careful hammering, giving it the form of

bars.

Why is the Peruvian steel so called?

Because it is an alloy of steel with certain portions of other metals from Peru. It is, technically speaking, sadder, not so easy to work as other steel, and yet much harder and tougher than any other.

CUTLERY.

Why is steel used for making cutting instruments? Because it combines the fusibility of cast with the malleability of bar iron, and when heated and suddenly cooled, becomes very hard.

The rapidity with which razors, knives, &c. are produced from the raw material, is truly astonishing. Thus in the workshops at Sheffield, we may in a few minutes see dinner knives made from the steel bar and all the process of hammering it into form, welding the tang of the handle to the steel of the blade, hardening the metal by cooling it in water and tempering it by de-carbonizing it in the fire.

The number of hands through which a common table knife passes in its formation is worthy of being known to all who use them. The bar steel is heated in the forge by the maker, and he and the striker reduce it in a few minutes into the shape of a knife. He then heats a bar of iron and welds it to the steel so as to form the tang of the blade which goes into the handle. All this is done with the simplest tools and contrivances. A few strokes of the hammer in connexion with some trifling moulds and measures, attached to the anvil, perfect, in two or three minutes the blade and its tang or shank. Two men, the maker, and striker, produce about nine blades in an hour, or seven dozen and a half per day. The rough blade thus produced, then passes through the hands of the filer, who files the blade into form by means of a pattern in hard steel. It then goes to the hafters to be hafted in ivory, horn, &c. and then to the finisher. In this profession, every table-knife, pocket-knife, or pen-knife, passes, step by step, through no less than 16 hands or 144 separate stages of workmanship.

Sheffield employed about 15,000 persons in these departments, four years since:

Besides those who are employed in Britannia-metal ware, smelting, optical instruments, grinding, polishing, &c. &c. making full 5,000 more. There are full 1,700 forges engaged in the various branches of the trades, and of course as many fires.

Why are the most minute instruments generally made with good steel?

Because it is much more ductile than iron: a finer wire being drawn from it than from any other metal.

Why is Wootz or Indian steel the most valuable for making edge tools ?

Because it is combined with a minute portion of the earths, alumina and silica; or rather perhaps, with the bases of these earths. Whether the earths are found in the ore, or are furnished by the crucible in making the steel, is not certainly known; nor is the Indian steel-maker probably aware of their presence. Wootz, in the state in which it is imported, is not fit to make into fine cutlery. It requires a second fusion, by which the whole mass is purified and equalized, and fitted for forming the finest edge instruments.— Brande.

Why does a razor operate best when dipped in hot waterp

Because the temperature of the blade has then been

raised, and the fineness of the edge proportionally increased.

In some experiments, the knife edges attached to the pendulum described by Captain Kater, in Phil. Trans. 1818, on being carefully hardened and tempered in the bath at 432o, were, on trial, found too soft. They were a second time hardened, and then heated to 212o, at which point the edges were admirably tempered. This, it will be remembered, is the heat of boiling water, and further illustrates the preceding question.

In the manufacture of a razor, it proceeds through a dozen hands; but it is afterwards submitted to a process of grinding, by which the concavity is perfected, and the fine edge produced. They are made from 1s. per dozen, to 20s. per razor, in which last the handle is valued at 16s. 6d.-Scissors, in like manner, are made by hand, and every pair passes through sixteen or seventeen hands, including fifty or sixty operations, before they are ready for sale. Common scissors are cast, and when riveted, are sold as low as 4s. 6d. per gross! Small pocket knives too are cast, both in blades and handles, and sold at 6s. per gross, or a halfpenny each! These low articles are exported in vast quantities in casks to all parts of the world.

ZINC.

Why is Zinc useful in the arts ?

Because, in combination with copper or tin, in various proportions, it forms some of the most useful compound metals or alloys. Thus, with copper, it constitutes brass, pinchback, and tombac; with little copper, Prince's metal;* with tin and copper, bronze.

Roofs covered with zinc are very numerous in the Low Countries, but have one bad quality. In cases of fire, the zinc being very combustible, soon becomes inflamed, and falling all around, occasions great danger

* See DOMESTIC SCIENCE, page 65.