Patent rough plate 1/8 and 1/4 inch thick, and 26 oz. sheet glass, are both made in panes containing up to 8 feet superficial. The perforations run across the width of the pane, and are useful for purposes of ventilation.

"There are two kinds of perforated glass : one having the perforations manufactured in the glass, the other having them afterwards cut. The latter is the best, as the former break very readily." 1

Cathedral Glass is generally rolled or sheet glass of a neutral tint. It is much used for ecclesiastical work.

Patent Rolled Cathedral is a species of thin rolled plate 1/8 inch thick, wavy on both sides, and tinted; and rolled white cathedral is of the same colour as ordinary glass without the lines.

Sheet Cathedral is also tinted and used for the same purposes. One variety has sand thrown upon its surface when hot, so that it fuses in, giving an appearance which is useful for artistic purposes. This is known as Sanded Sheet Cathedral.

Ground Glass, or Obscured Glass, has one side covered with an opaque film, formed either by grinding the surface or by melting powdered glass upon it.

The names for this glass seem to be used indiscriminately, without reference to the process by which it is made. Such glass is useful wherever light is required without transparency.

Enamelled Glass is obscured in parts to a design which is stencilled upon it. Powdered glass, or enamel, is placed so as to form the pattern, and is then fluxed in by heat as before.

Stained Enamelled Glass is made as follows : - The whole is first covered with enamel; the parts to be coloured are rubbed off with the aid of stencil plates, and then treated with chemical substances; these, when subjected to the heat of the kiln, produce the colour required.

Embossed Glass is also obscured in parts so as to form a pattern, as follows : - The design is drawn or stencilled on the glass to be enamelled, and the remainder of the surface covered with Brunswick black. The whole is then covered with fluoric acid, which eats into the unprotected portions, obscuring them in the form of the pattern drawn.

1 Seddon.

Coloured Glass can be made in every variety of tint by adding metallic oxides and other substances to the materials before fusion.

Flashed Colours are those in which plain sheet glass is covered on one side only with a thin layer of coloured glass.

Designs may be formed in this glass by eating off the coloured layer, where it is not required, with fluoric acid.

Pot Metals are those in which the glass is coloured throughout its thickness.

Special hinds of Glass are made for painted windows and other work of an artistic kind, but the description of such glass falls outside the scope of these Notes.

Glass Tiles are made both in rough plate and sheet glass, either plain, fluted, or to correspond with the various shapes of earthenware tiles, so as to be worked in with them in roofs, and admit light without the expense of skylights, etc.

Glass Slates are also made both in rough plate and in sheet glass : the former in thicknesses from 1/8 to 1/2 inch; the latter of glass varying from 16 to 32 oz. The areas of the glass slates correspond with those of ordinary building slates as given at p. 27.

Interception Of Light By Glass

The effect of different descriptions of glass on the diminution of light has been shown by experiment1 to be as follows : -

British polished plate 1/4 inch thick intercepts 13 per cent of the light. Bough-cast plate „ „ 30 ,.

Do., rolled, four flutes to an inch „ 53 „

Sheet glass, 32 oz. . . „ 22 „

1 Galton.