GLASS, WATER 677 



the brass moulds atvJ turned. Sometimes, also, lead moulds are used. After the two 

 moulds are made, they are ground face to face with fine emery. 



The piece of glass is now roughed into a circular form by a pair of pincers, leaving 

 it a little larger than the finished lens ought to be, and then smoothed round upon 

 the stone disc, or in an old mould with emery and water, and is next made fast to a 

 holdfast. This consists of a round brass plate, having a screw in its back ; and is 

 somewhat smaller in diameter than the lens, and two-thirds as thick. This is turned 

 concave upon the lathe, and then attached to the piece of glass by drops of pitch 

 applied to several points of its surface, taking care, while the pitch is warm, that the 

 centre of the glass coincides with the centre of the brass plate. This serves not 

 merely as a holdfast, by enabling a person to seize its edge with the fingers, but it 

 prevents the glass from bending by the necessary pressure in grinding. 



The glass must now be ground with coarse emery upon its appropriate mould, 

 whether convex or concave, the emery being all the time kept moist with water. To 

 prevent the heat of the hand from affecting the glass, a rod for holding the brass 

 plate is screwed to its back. For every six turns of circular motion, it must receive 

 two or three rubs across the diameter in different directions, and so on alternately. 

 The middle point of the glass must never pass beyond the edge of the mould ; nor 

 should strong pressure be at any time applied. "Whenever the glass has assumed the 

 shape of the mould, and touches it in every point, the coarse emery must be washed 

 away, finer be substituted in its place, and the grinding be continued as before, till all 

 the scratches disappear, and a uniform dead surface be produced. A commencement 

 of polishing is now to be given with pumice-stone powder. During all this time the 

 convex mould should be occasionally worked in the concave, in order that both may 

 preserve their correspondence of shape between them. After the one surface has 

 been thus finished, the glass must be turned over, and treated in the same way upon 

 the other side. 



Both surfaces are now to be polished. "With this view equal parts of pitch and resin 

 must be melted together, and strained through a cloth to separate all impurities. The 

 concave mould is next to be heated, and covered with that mixture in a fluid state to 

 the thickness uniformly of one quarter of an inch. The cold convex mould is now to 

 be pressed down into the yielding pitch, its surface being quite clean and dry, in order 

 to give the pitch the exact form of the ground lens ; and both are to be plunged into 

 cold water till they be chilled. This pitch impression is now the mould upon which 

 the glass is to be polished, according to the methods above described, with finely- 

 washed colcothar and water, till the surface become perfectly clear and brilliant. To 

 prevent the pitch from changing its figure by the friction, cross lines must be cut in 

 it about half an inch asunder, and one-twelfth of an inch broad and deep. These 

 grooves remove all the superfluous parts of the polishing powder, and tend to pre- 

 serve the polishing surface of the pitch clean and unaltered. No additional colcothar 

 after the first is required in this part of the process, but only a drop of water from 

 time to time. The pitch gets warm as the polishing advances, and renders the friction 

 more laborious from the adhesion between the surfaces. No interruption must now 

 be suffered in the work, nor must either water or colcothar be added ; but should 

 the pitch become too adhesive, it must be merely breathed upon till the polish be 

 complete. The nearer the lens is brought to a true and fine surface in the first grind- 

 ing, the better and more easy does the polishing become. It should never be submitted 

 to this process with any scratches perceptible in it, even when examined by a magnifier. 



As to small lenses and spectacle eyes, several are ground and polished together. 

 The pieces of glass are affixed by means of a resinous cement to the mould, close to 

 each other, and are then all treated as if they formed but one large lens. Plane 

 glasses are ground upon a surface of pitch rendered plane by the pressure of a piece 

 of plate glass upon it in its softened state. 



Lenses are also ground and polished by means of machinery, into the details of 

 which the limits of this work will not allow us to enter. See LENSES. 



(For the Importations of glass see Table at top of next page.) 



GLASS PAPER and CLOTH. Paper and cloth being covered with glue, 

 sand, varying in its degree of fineness, is dusted over it, and of course adheres. 

 These are used for polishing, or removing the rough surfaces of woods and metals. 



GXiASS, WATER. The term ' water-glass ' is applied to those alkaline sili- 

 cates which, by containing an excess of alkali, are soluble in water. A potash water- 

 glass may be prepared by melting together 45 parts of silica, 30 of potash, and 3 of 

 carbon, in the form of pulverised wood-charcoal. Soda water-glass may be obtained 

 by fusing a mixture of 45 parts of silica, 23 of calcined soda, and 3 of charcoal. 

 "Water-glass is used for rendering wood and textile fabrics uninflammable ; it is em- 

 ployed in the preparation of Eansome's artificial stone, and was introduced by Von 

 Fuchs in his stereochromatic wall-painting. See STEBEOCHBOMY ; STONE, ARTIFICIAL. 



