sand, flints, spar, or some other silicious matters. White sand is the substance in the most repute at present, as it requires no preparation for coarse goods; and for the finest, washing in fair water is sufficient: whereas flints require a tedious process of calcination, and after that to be pulverized. Many other substances may be used for experiment; though sand only is employed in the manufactory.

It is also necessary that the silicious matter should be fused in contact with something called a flux. The substances proper for this purpose are lead, borax, arsenic, nitre, or any alkaline matter. The lead is used in the state of red lead, and the alkalics are soda, pearl-ashes, sea salt, and wood-ashes. When red lead is used alone, it gives the glass a yellow cast, and requires the addition of nitre to correct it. Arsenic, in the same manner, if used in excess, is apt to render the glass milky. For a perfectly transparent glass, the pearl-ashes are found much superior to lead; perhaps better than any other flux, except it be borax, which is too expensive to be used, except for experiments, or for the best looking-glasses.

The materials for making glass must first be reduced to powder, which is doue in mortars or by horse-mills. After sitting out the coarse parts, the proper proportions of silex and flux are mixed together and put into the calcining furnace, where they are kept in a moderate heat for five or six hours, being frequently stirred about during the process. When taken out the matter is called frit. Frit is easily converted into glass by only pounding it, and vitrifying it in the melting pots of this glass furnace: but in making fine glass it will sometimes require a small addition of flux to the frit to correct any fault. For, as the flux is the most expensive article, the manufacturer will rather put too little at first than otherwise, as he can remedy this defect in the melting pot. heat in the furnace must be kept up until the glass is brought to a state of perfect fusion; and during this process any scum which arises must be removed by ladles. When the glass is perfectly melted, the glass-blowers commence their opera

tions.

The

The following composition of the ingredients for glass are extracted from the Handmaid to the Arts:

For the best flint glass, 120lbs. of white sand. Julbs, of red lead, 40lbs. of the best pearlashes, 20lbs. of nitre, and five ounces of magnesia: if a pound or two of arsenic he added, the composition will fuse much quicker, and with a lower temperature.

"For a cheaper flint-glass, 120lbs. white sand, 35lbs. of pearl-ashes, 40lbs. red lead, 13lbs. of nitre, six pounds of arsenic, and four ounces of magnesia.

"This requires a long heating to make clear glass; and the heat should be brought on gradually, or the arsenic is in danger of subliming before the fusion commences. A still cheaper composition is made by omitting the arsenic in the foregoing, and substituting common sea salt.

"For the best German crystal glass, 120lbs. of calcined flints or white sand, the best pearl-ashes 70lbs., saltpetre 10lbs., arsenic half a pound, and tise ounces of magnesia. Or, a cheaper composition for the same purpose is, 120lbs. of sand or Hints, 46lbs. of pearl-ashes, seven pounds of nitre, six pounds of arsenic, and tive ounces of magneThis will require a long continuauce in the

furnace; as do all others where much of the arsenic is employed.

“For looking-glass plates, washed white sand 60lbs., purified pearl-ashes 25lbs., nitre 15lbs., and seven pounds of borax. If properly managed, this glass will be colourless. But if it should be tinged by accident, a trifling quantity of arsenic, and an equal quantity of magnesia, will correct it; an ounce of each may be tried first, and the quantity increased if necessary.

"The ingredients for the best crown-glass must be prepared in the same manner as for lookingglasses, and mixed in the following proportions: 60ibs. of white sand, 30lbs. of pearl-ashes, fand 15lbs. of nitre, borax a pound, and half a pound of arsenic.

"The composition for common green windowglass is 120lbs. of white sand, 30lbs. of unpurified pearl-ashes,wood-asheswellburnt and sifted 60lbs. common salt 20lbs., and five pounds of arsenic.

"Common green bottle-glass is made from 200lbs. of wood-ashes, and 10015s. of sand; or 170lbs. of ashes, 100lbs. of sand, and 50lbs. of the lava of an iron-furnace: these materials inust be well mixed."

The materials employed in the manufactory of glass, are by chemists reduced to three classes, namely, alkalies, earths, and metallic oxyds.

The fixed alkalies may be employed indifferently; but soda is preferred in this country. The soda of commerce is usually mixed with common salt, and combined with carbonic acid. It is proper to purify it from both of these foreign bodies before using it. This, however, is seldom done.

The earths are silica, lime, and sometimes a little alumina. Silica constitutes the basis of glass. It is employed in the state of fine sand or flints; and sometimes, for making very fine glass, rock crystal is employed. When sand is used, it ought if possible to be perfectly white; for when it is coloured with metallic oxyds, the transparen of the glass is injured. Such sand can only be employed for very coarse glasses. It is necessary to free the sand from all the loose earthy particles with which it may be mixed, which is done by washing it well with water.

Lime renders glass less brittle, and enables it to withstand better the action of the atmosphere. It ought in no case to exceed the twentieth part of the silica employed, otherwise it corrodes the glass pots. This indeed may be prevented by throwing a little clay into the melted glass; but in that case a green glass only is obtained.

The metallic oxyds employed are the red oyyd of lead or litharge, and the white oxyd of arsenic. The red oxyd of lead, when added in sufficient quantity, enters into fusion with silica, and forms a glass without the addition of any other ingredient. Five parts of minium and two of silica form a glass of an orange-colour and full of striæ. Its specific gravity is five. The red oxyd of lead renders glass less brittle and more fusible; but, when added beyond a certain proportion, it injures the transparency and the whiteness of glass.

The white oxyd of arsenic answers the same purposes with that of lead; but on account of its poisonous qualities it is seldom used. It is customary to add a little nitre to the white oxyd of arsenic, to prevent the heat from reviving it, and rendering it volatile. When added beyond a certain proportion, it renders glass opaque and milky like the dial-plate of a watch. When any com