Lustible body is present, it is usual in some manufactures to add a little white oxyd of arsenic. This supplying oxygen, the combustible is burnt, and flies off; while the revived arsenic is at the same time volatilized.

There are several kinds of glass adapted to different uses. The best and most beautiful are the flint and the plate glass. These, when well made, are perfectly transparent and colourless, heavy and brilliant. They are composed of fixed alkali, pure silicious sand, calcined flints, and litharge, in different proportions. The flint-glass contains a large quantity of oxyd of lead, which by certain processes is easily separated. The plate-glass is poured in the melted state upon a table covered with copper. The plate is cast half an inch thick, or more, and is ground down to a proper degree of thinness, and then polished.

Crown-glass, that used for windows, is made without lead, chiefly of fixed alkali fused with silicious sand, to which is added some black oxyd of manganese, which is apt to give the glass a tinge of purple.

Bottle-glass is the coarsest and cheapest kind: into this little or no fixed alkali enters the composition. It consists of an alkaline earth combined with alumina and silica. In this country it is composed of sand and the refuse of the soap-boiler, which consists of the lime employed in rendering his alkali caustic, and of the earthy matters with which the alkali was contaminated. The most fusible is flint-glass, and the least fusible is bottleglass.

Flint-glass melts at the temperature of 10° Wedgewood; crown-glass at 30°; and bottleglass at 47°. The specific gravity varies between 2:48 and 3:33.

Glass is often tinged of various colours by mixing with it while in fusion soine one or other of the metallic oxyds; and on this process, well conducted, depends the formation of pastes or factitious gems.

Blue glass is formed by means of oxyd of co

balt.

Green, by the oxyd of iron or of copper. Violet, by oxyd of manganese.

glass is termed blowing, from its being in a great measure performed by the operator blowing through an iron tube, and by that means inflating a piece of glass which is heated so as to become soft and exceedingly pliable. By a series of the most simple and dextrous operations, this beautiful material is wrought into the various utensils of elegance and utility, by methods which require but very few tools, and those of the most simple construction.

The glass-blowers' furnace is of a circular form, as shewn in the plan, fig. 2, Plate 82. It consists of three distinct parts. The lowest is a large arch, which is carried beneath the centre of the furnace: in the plan, fig. 2, this is represented by the dotted lines AA: in the section, fig. 1, nothing of this arch is seen, except part of its upright sides AA. In the centre of the furnace the covering of this arch is wanting, and its place is supplied by a grate, (represented in the plan) upon which the fire is made. The arch AA, which is called the draught arch, is intended to bring a constant supply of fresh air to the furnace. The second part of the furnace is a circular wall KK, of masonry or brick work, strengthened by nine ribs or piers BBB, which extend from the foundation to the top of the furnace, (as shewn in the section). Within the circular wall or waist of the furnace, the crucibles or pots to contain the glass are placed; these are nine in number, and are situated behind the spaces between each pier. The fire is made upon the grate in the centre of the furnace, and its flames are reverberated down upon the pots by a dome DD, fig. 1, called the vault, constructed of firebricks. The vault, and indeed the whole superstructure of the furnace, is supported only by the nine piers B: by this means nine apertures are left beneath the vault which are the mouths of the furnace.

The vignette at the top of Plate 81, is a view of the interior of a glass-house, with workmen performing the various operations. In this figure, the nine mouths of the furnaces are represented as partially closed by a screen of fire-bricks, in which are three apertures to give the workman access to the pots; the use of the screen is to de

Purple, by the purple oxyd of gold. White, by the oxyd of arsenic and of zinc. Yellow, by the oxyd of silver and by combustible bodies.

Opticians, who employ glass for optical instruments, often complain of the many defects under which it labours. The chief of these are the following:

Streaks. These are waved lines, often visible in glass, which interrupt distinct vision. They are probably owing sometimes to want of complete fusion, which prevents the different materials from combining sufficiently; but in some cases also they may be produced by the workmen lifting up, at two different times, the glass which is to go to the formation of one vessel or instrument.

Tears. These are white specks or knots, occasioned by the vitrified clay of the furnaces, or by the presence of some foreign salt.

heat of the furnace; and the apertures are there. fore proportioned to the size of the work to be performed. The nine pots are placed exactly beneath the mouths of the furnace, and are arranged round the furnace upon a circular course of brick. work (EE in the clevation), so that the current of flame reflected from the vault DD, strikes directly upon them. The flame and heated air are carried off from the furnace by nine flues, five of which FFF, are seen in fig. 1, Pl. 82, into an upper dome GG, which is the third part. It has a cylindric chimney HH, erected on the top of it, and carried up some height, to cause sufficient draught for the fire.

Bubbles. These are air-bubbles which have not been allowed to escape. They indicate want of complete fusion, either from too little alkali, or the application of too little heat.