cible till nearly solid, or else be removed from the crucible in the liquid state, being gathered in successive layers on the end of an iron rod. If the glass is allowed to solidify in the crucible, the mass may be afterwards broken by a hammer into conchoidal lumps, to which a lenticular shape may be imparted by exposing them to a softening heat in a reverberatory furnace. The temperature of this furnace should be very equable, and not above dull redness. A method has been contrived for cleaving the mass of glass while cooling in the crucible, in such a manner that the fracture follows the direction of the more faulty parts; in which way masses of homogeneous glass weighing as much as forty pounds are said to have been obtained.

After several years spent in laborious experiments, the late M. Guinand, of Brennets, an inconsiderable village among the mountains of Neufchâtel in Switzerland, appears to have discovered some method of procedure, whereby homogeneous pieces of flint-glass, capable of forming lenses a foot in diameter, could be procured almost with certainty. An eminent French optician states that eight or nine out of ten object-glasses made of M. Guinand's flint-glass proved of excellent quality, but only one or two out of the same number made of glass from the English and French makers came up to the test. The process discovered by M. Guinand has not been made public, but the secret is said to be in the possession of his son and M. Bontemps, and the latter has made some important improvements on the process. A specimen of M. Guinand's flint-glass, of specific

gravity 3.616, has been analysed with the following results silicic acid 44·3; oxide of lead 43·05; potash 11-75. These proportions nearly correspond with three equivalents of potash, five equivalents of oxide of lead, and eighteen equivalents of silicic acid. It is said that the freedom of the glass from bubbles depends, in a great measure, on the rigour observed in compounding the materials in the proper proportions, as well as from certain precautions in the management of the fire towards the end of the operation.

The results of a series of well-conducted experiments by Mr. Faraday, having for their object the discovery of a process for preparing homogeneous glass with some certainty, form the subject of the Bakerian Lecture in the Philosophical Transactions for 1830. The glass recommended by Mr. Faraday is a combination of silicate of lead with borate of lead, the materials being perfectly pure, and vitrified in a platinum crucible. If the air-bubbles are disengaged but slowly, it is advised to throw into the crucible a little spongy platinum, in fine powder, which should be stirred about with a platinum spatula. The powder readily subsides, and the glass may then be poured off quite clear and free from striæ. This glass is considered to be less attractive of moisture than most other kinds. From the limited scale on which this process would have to be performed, the suggestions of Mr. Faraday have not been carried out by manufacturers.

It may be observed, that an achromatic object-glass for a telescope or microscope, that is, an object-glass which does not afford coloured fringes around the

edge of the image, distinguished as chromatic aberration, must consist of two lenses made of different kinds of glass, differing in the proportion which their refractive power bears to their dispersive power. Flint-glass and crown-glass are well adapted for being formed into such a compound lens, the dispersive power of the former being nearly double that of the latter, while the mean refractive powers of the two kinds are nearly the same.

§ VI. COLOURING OF GLASS.

When certain metallic oxides are mixed with glass in a fluid or semi-fluid state, an intimate combination takes place between the glass and the metallic oxide, with the production of coloured compounds, which become dissolved and retained in the mass of glass without rendering it opaque. All the varieties of glass, and indeed, most saline bodies which possess a vitreous character, as borax and biphosphate of soda, are susceptible of being coloured in this manner by metallic oxides, but in many cases particular oxides are found to produce the richest colours with a particular kind of glass. Most of the colouring oxides afford more brilliant tints with the clear and white glass made of potash and lime, than with glass containing oxide of lead; but in a few cases, lead glass is decidedly preferable to potash and lime glass. The oxides which are mostly employed as vitrifiable colouring matters are the following black and red oxides of copper, oxide of cobalt, oxide of iron, oxide of gold, oxide of man

ganese, oxide of chromium, oxide of silver, oxide of platinum, oxide of uranium, and oxide of antimony.

These colouring matters are not always introduced into the glass in the form of oxides. Thus silver is sometimes applied as the chloride, and sometimes in the metallic state. But in whatever form the metal is applied, it seems to exist in the coloured glass as the oxide, in combination with silicic acid, probably as a double silicate, the other base being one of those proper to the glass. Certain metallic oxides which are decomposed by a comparatively slight elevation of temperature into oxygen gas and their metallic bases, resist decomposition at all attainable temperatures in contact with melted glass; and not only so, but the metal itself, if applied to the surface of the melted glass, becomes converted into an oxide by the acquisition of oxygen from the air. This is the case, for example, with silver.

The art of colouring glass is so comprehensive, in its details, that nothing more than a few general observations on the materials employed, and the ordinary modes of applying them, can be included in the present article. It appears to have been prosecuted at a period almost as early as that of the manufacture of glass itself. The earthenware beads with which some Egyptian mummies are adorned, are covered with a true glass coloured by means of a metallic oxide; and small pieces of transparent glass of a turquoise colour have lately been discovered among the tombs at Thebes, which are supposed to have been used as the glaze for beads. Both the black oxide (protoxide), and the red

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oxide (suboxide) of copper, are used as colouring materials for glass. The protoxide imparts either a bright blue or a full green, according to the manner in which it is applied. This colouring material is usually introduced in the form of oxide, obtained by heating pieces of sheet copper to redness with exposure to the air, and quenching them in cold water in order to detach the scales of oxide. more convenient form is the subcarbonate, prepared by precipitation from a solution of the sulphate, with carbonate of soda. The acetate of copper is also sometimes used with advantage. To produce a full green glass the vitreous basis should contain some oxide of lead (though this is not essential to the production of the colour), and the oxide may be employed in any proportion between five and fifteen per cent. It is said that a beautiful blue glass may be obtained by means of the double subcarbonate of copper and ammonia, precipitated from the solution of the nitrate of copper by carbonate of ammonia. The black oxide of copper has been found to be the only colouring matter in some specimens of Egyptian blue glass, analysed by Sir H. Davy and M. Chaptal; but in other similar specimens, the oxide of copper is found to be accompanied by oxide of iron. The suboxide of copper tinges glass of a beautiful ruby-red, instead of a green or blue, but that colouring matter is very difficult of application. The intensity of the colour is increased by the addition of peroxide of iron, which imparts, of itself, a red colour to glass, but not so deep and brilliant as that of the suboxide of copper. One process for preparing this ruby-glass consists in mixing cream of