PLATINUM.

Why is platinum so called?

Because it resembles silver; the term, in the language of Peru, meaning "little silver."

Platinum may always be known from other metals by its superior specific gravity, it being the heaviest body in nature. The important uses to which this precious metal may be applied, can be easily conceived, when it is considered that it unites the indestructibility of gold to a degree of hardness almost equal to that of iron; that it resists the action of the most violent fire, and also of the most concentrated acids. One of its more useful applications has been to the coating of copper with as much ease as the common operation of tinning. The high value of platinum is however very much against its general adoption, for although much cheaper than gold, it is worth between four and five times it weight of silver.

Why is the alloy of steel and platinum well adapted for mirrors?

Because it takes a fine polish, and does not tarnish. Why is platinum well adapted for the manufacture of rings and chains?

Because it has the property of being united by welding, either one piece to another, or with iron, or steel. Hence its durability must add to its value.

A beautiful coinage of platinum has lately been issued in Russia, the metal being found in considerable quantities in the Uralian mountains of Siberia.

Why is rhodium used for the nibs of metallic pens? Because of its extreme hardness and durability. For this successful application, the elegant arts are indebted to the suggestion of the late Dr. Wollaston

GLASS.

Why is lead important in glass ?

Because its oxide, in the form of litharge, or minium, increases the fusibility of the compound, gives it greater tenaciousness when hot, increases its refrac

tive power, and enables it to bear sudden changes of temperature. It is a copious ingredient in the London flint glass, celebrated for its brilliancy when cut, and used for most optical purposes. Lead, however, renders glass so soft as easily to scratch.

The manufacture of glass is as follows:-The glasspots are placed round a dome-shaped furnace, built upon arches, and open beneath for the free admission of air; there are generally six in each furnace, and they are entirely enclosed, except at an orifice on the side opening into a small recess formed by the alternate projection of the masonry and the flues, in which the workmen stand. Coal is the fuel employed, and the furnace is so built that a rapid current of flame may be directed round each glass-pot, which afterwards passes out with the smoke into the dome and chimney, heating a broad covered shelf in its passage, which is the annealing oven. The materials, or fret, being fused, and the impurities removed, gradually become clearer, abundance of air-bubbles are extricated, and at length the glass appears uniform and complete; the fire round each individual pot is then damped, till its contents acquire a consistency fit for working; the whole process requiring about forty-eight hours from the time the pots are filled. At the working heat, which is a full red, the glass has a peculiarly, tenacious consistency, and as it adheres but feebly to polished metals, it is easily wrought and managed with iron tools.*

* Mr. Brande, in a note to his Manual of Chemistry, observes: "All common glass, when reduced to a fine powder, is more or less acted on by boiling water, which separates the alkali, and its entire disintegration seems only to be prevented by the insolubility of the silica. Glass which has long been exposed to the weather, frequently exhibits a beautiful irridescent appearance, and is so far decayed, that it may be scratched with the nail. Several years ago, I examined some bottles of wine which had lain in a wet cellar, near the Bank, upwards of 150 years, having been deposited there (as circumstances proved) previous to the great fire of London in 1666. The glass was soft, and greatly corroded upon the surface, in consequence of the abstraction of its alkali. The wine appeared to have been Malaga and Claret; the latter had perished, but the former was still vinous."

It is a curious fact in the history of discovery, that the manufacture of glass is unknown at Sidon, though this part of the country was once famous for the discovery of, as well as production of articles in, that material. The story of the discovery of glass by Phœnician mariners at Belus, near Sidon, in Syria, is mentioned by Pliny. Dyeing, however, is still practised, though not with the same success as among the ancient Tyrians, who were descended from the builders of Sidon. The standard of Syria in arts and manufactures, is, indeed, every where, much below that of the most backward nation in Europe.

Why do all glass articles require to be carefully annealed, or suffered to cool very slowly?

Because they would otherwise be remarkably brittle, and apt to crack, and even fly into pieces upon the slightest touch of any hard substance.

Why are plate-glass windows superior to those of common glass P

Because the two surfaces of common window-glass are not perfect planes, and not perfectly parallel to each other, as in the case of plate-glass; whence objects seen through the former, appear generally more or less out of shape; and hence comes the elegance and beauty of plate-glass windows; and the singular distortion of things viewed through that swelling or lump of glass which remains where the glass-blower's instrument was attached, and which appears at the centre of certain very coarse panes.

The variation is also thus philosophically explained: "As it is the obliquity between the passing ray and the surface, which, in any case of refraction, determines the degree of bending, a body seen through a medium of irregular surface, appears distorted, according to the nature of that surface."-Arnott.

Why is plate-glass so expensive?

Because of the difficulty of producing a perfect

plate, without specks, bubbles, or waves, and the risk of breakage. Its manufacture is as follows:

The materials being fused, are poured upon a hot copper-plate; the mass is then rolled out, annealed, and afterwards polished by grinding with sand, emery, and colcothar.

Previously to 1559 all the glass employed in mirrors was blown; when a Frenchman, named Thevart, discovered the art of casting it. The art of polishing it was invented by Rivière Dufresny. The French glass for the royal manufactories is now cast at Tourlaville, near Cherbourg, and at St. Gobin an ancient chateau near la Fere; it is afterwards sent to Paris to be polished, silvered, and cut. Glasses are finished here to the value of nearly 600l. each, and are sometimes 10 feet in height by 63 in width.

The price of large-sized pier-glasses is within the reach of the most moderate incomes, and there is scarcely a family in France which does not possess one or two; while, as we all know, they are rarely to be met with in England, except in expensively furnished houses.

Why was the black oxide of manganese formerly called glass soap?

Because it cleanses certain impurities, and especially the green tinge which is apt to arise from impure alkali.

Why do glass-cutters use the point of a diamond for dividing and shaping their panes ?

Because diamond is the hardest of known substances, and cuts or scratches every other body.-Arnott.

Glass can, however, only be cut by the natural point of the diamond. There are various other methods of cutting glass, among which the following, from a French Journal, merits notice.

If a tube, or goblet, or other round glass body is to be cut, a line is to be marked with a gun-flint, having

a sharp angle, an agate, a diamond, or a file, exactly on the place where it is to be cut. A long thread, covered with sulphur, is then to be passed two or three times about the circular line, and is to be inflamed and burnt; when the glass is well heated, some drops of cold water are to be thrown on to it, when the pieces will separate in an exact manner as if cut with scissors. It is by this means that glasses are cut circular into thin bands, which may either be separated from, or repose upon, each other at pleasure, in the manner of a spring.

Why is the white enamel in imitative gold trinkets of such little value?

Because it is merely glass rendered more or less milky or opaque, by the addition of oxide of tin; it forms the basis of many of the coloured enamels, which are tinged with the metallic oxides.

Why are leaden bullets let fall on glass cooled in the open air, without breaking it, whereas a few grains of sand also let fall on the glass would break it into a thousand pieces?

Because the lead does not scratch the surface of the glass; while the sand, being sharp and angular, scratches it sufficiently to break it.

Why are glass-house cinders, or clinkers, employed in hydraulic mortar?

Because they contribute to its rapid consolidation. This advantage was employed in the construction of the bridge of Louis XVI. at Paris, and in building the jetties and fort at Cherbourg.

POTTERY.

Why is common clay of a brownish colour? Because of the oxide of iron which it contains. Why is nickel prized by the French manufacturers of porcelain?

Because its oxide affords them a very delicate grass