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raised, and the fineness of the edge proportionally in

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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 432°, were, on trial, found too soft. They were a second time hardened, and then heated to 212, 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 18. 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.

to those who approach the building. In short, zinc is the most combustible metal we have. If beaten out into thin leaves, it will take fire from the flame of a common taper.

Why has the oxide of zinc been substituted for white lead in house-painting.

Because it preserves a good colour much longer: it is not, however, of so perfect a white as lead.

TIN.

Why did the ancients mix tin with their copper coins and edge tools?

Because it occasioned the coins to wear longer, and it imparted sufficient hardness to the copper to render it capable of forming very good cutting instruments. Mr. Parkes, in analysing several Roman brass coins, from various periods of the Empire, found tin to be a component part in all of them.

Why is not Spanish tin used in this country?

Because it bears a prohibitory duty of 30l. per cent. It is raised in great quantities in South America, and is very pure, but not so neatly manufactured as the Cornish tin. According to Aristotle, the tin mines of Cornwall were known and worked in his time. Diodorus Siculus, who wrote 40 years before Christ, describes the method of working these mines, and says, that their produce was conveyed to Gaul, and thence to different parts of Italy. The miners of Cornwall were so celebrated for their knowledge of working metals, that about the middle of the 17th century, the renowned Becher, a Physician of Spire, and tutor of Stahl, came over to this country to visit them.

A celebrated tin mine was the famous wherry mine, near Penzance. The shaft through which the miners went down to work, was situated nearly 100 yards below water mark. "The opening of this mine" says Dr. Maton, "was an astonishingly adventurous undertaking. Imagine the descent into a mine through the sea, the miners working at the depth of 17 fathoms

below the waves; the rod of a steam engine, extending from the shore to the shaft, a distance of nearly 120 fathoms, and a great number of men momentarily menaced with an inundation of the sea, which continually drains in no small quantity through the roof of the mine, and roars loud enough to be distinctly heard in it." The working of this mine was wholly given up in the year 1798.

Such is the mineral wealth of Cornwall, that it contains more men, who possess fortunes, sprung from the mines, of five and from that to twenty thousand pounds, than there are in any other county of England, excepting the metropolis and its vicinity; and there are some instances of individuals acquiring from fifty to two hundred thousand pounds, from the mines, and by a fortunate course of trade.

Why should tin be chosen for its lightness?

Because its purity is in exact ratio with its levity; while gold, on the contrary, unless alloyed with platinum, is fine in proportion to its density.

Why is tin so important to the dyer?

Because it is employed to give a brightness to cochineal,* archil, and other articles used in forming reds and scarlets; and to precipitate the colouring matter of other dyes. For these purposes it is previously dissolved in a peculiar kind of aqua-fortis, called dyers' spirit.

Tin is consumed in large quantities by the dyers; it is also used for covering sheet iron to prevent its rusting, and in forming plumbers' solder, speculum metal, pewter, and some other alloys. Its oxides are used in polishing glass, in glazing some kinds of earthenware, &c.

Why is tin-plate so called?

Because it is made by dipping clean iron plates into melted tin. When tin-plate is washed over with

*See ZOOLOGY-Insects, page 258.

a weak acid, the crystalline texture of the tin becomes beautifully evident, forming an appearance which has been called moire metallique.

Why are pins whitened by boiling in grain-tin and supertartrate of potash?

Because the tartaric acid first dissolves the tin, and then gradually deposits it on the surface of the pins, in consequence of its greater affinity for the zinc, of which the brass wire is composed.

Why were the Stannary Courts so called?

Because they regulated the affairs of the tin (Stannum, Latin,) mines, and determined causes among the tinners, whether criminal or actions for debt. At Lydford, on the borders of Dartmoor, was one of the Stannary prisons: hence the Devon and Cornwall saying: "First hang and draw,

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Then hear the cause by Lydford Law; or Lydford Law, by which they hang men first, and try them afterwards.

LEAD.

Why is lead employed in refining the precious metals? Because when mixed with them in a great heat, it rises to the surface combined with all heterogeneous matter. Lead is employed to cover buildings, to form water-pipes, (though Vitruvius, the Roman architect, in the time of Augustus, condemned this practice,) and to make a great variety of vessels for economical purposes. Its oxides are used for dyeing and calico-printing, in the manufacture of glass, earthenware, and porcelain and lead is capable of forming various alloys. There is also a large consumption of lead in making shot.*

Why is lead employed in the manufacture of white metal buttons ?

Because it has been discovered that a certain pro

* See MECHANICS, p. 20.

portion of lead may be mixed with the metal formerly used, without injuring the appearance of the button; thus affording a very considerable additional profit to the manufacturer.

Why is lead employed to correct harsh wine?

Because it has the property of imparting a saccharine taste when dissolved in acids, as in that of the wine. The ancients knew that this metal rendered harsh wine milder, but it was not universally known to be poisonous. According to Pliny, the Greeks and Romans proved the quality of their wines by dipping a plate of lead in them. Lead will also take off the rancidity of oils.

Why were blocks of lead called pigs?

Because they might be distinguished from larger blocks, called sows, which latter term is still retained in the word sow-metal. In 1773, a pig of lead was dug up near Tamworth, with an inscription of the date 76, A.D., or 1697 years old; thus proving lead to have been used by the Romans in this country.

Why is lead cast in such regular sheets?

Because the melted metal is suffered to run out of a box through a long horizontal slit upon a table prepared for the purpose, while the box is drawn by appropriate ropes and pulleys along the table, leaving the melted lead behind it in the desired form, to congeal. The lead thus cast is then passed between two iron rollers placed at such a distance from each other, as will reduce it to the requisite thickness.

Why is common lead changed into red lead by melting it in ovens with a free access of atmospheric air?

Because the lead absorbs so much oxygen as to become converted into an oxide. Thus, the melted lead is exposed until the surface is covered with a pellicle; this pellicle being removed, another is formed; and thus, by successively removing the pellicle as it forms, the greater part of the lead is soon changed into a yel

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