Glass may be written on for temporary purposes, by French chalk; pencils of this substance will be found convenient. Glass may be written on with ink. if the surface is clean and dry, and the pen held nearly perpendicular. The shell-lac ink (see Ink) is the best for labels, as it resists damp, etc. To scratch glass, a scratching diamond is used; or a piece of flint, or crystal of quartz, or the point of a small 3-square file. To engrave on glass, fluoric (hydrofluoric) acid is used, either in the liquid state or in vapour. The glass must be warmed, and coated with wax, or engravers' cement, and the writing or design traced through the wax with a bradawl, or other pointed instrument. The liquid fluoric acid is poured on it, and left to act on the uncovered portions of the glass; or the fluorspar may be powdered and made into a paste with oil of vitriol, and laid over the prepared surface, and covered with lead-foil or tea-lead: or bruised fluorspar is put in a Wedgewood evaporating basin, with sufficient oil of vitriol to form a thin paste, and the prepared glass laid over the basin, so that the vapours may act on the portions from which the wax has been removed. To cut glass (besides the usual method of dividing cut glass by a glazier's diamond), the following means may be used: - To divide glass tubes or rods, form a deep mark round them with the edge of a sharp 3-square file, then with a hand placed on either side of the mark, break the rod with a slightly stretching as well as bending motion. A diamond or sharp flint may be substituted for a file. Flasks, globes, and retorts may be divided by means of iron rings, having a stem fixed in a wooden handle. Make the ring red-hot, and apply it to the flask, etc. If the vessel do not break where it came in contact with the ring, wet the part, and it will generally separate. Another method is to twist together 2 or 3 threads of cotton, such as is used for wicks, moisten them with spirit of wine, and encircle the flask with them; then holding the flask horizontally, set fire to the wick, and turn the flask with the fingers, so as to keep the flame in the direction of the thread. If the separation does not take place the first time, the process may be repeated after the glass has cooled. By these means a common oil-flask may be divided into an evaporating-dish and a funnel. By means of a stout iron rod, fixed in a wooden handle, and terminating in a blunt point, and heated to redness, broken retorts, globes, and flasks may be converted into useful evaporating dishes, etc. If any crack exist, it may easily be led in any direction, as it will follow the motion of the heated iron. If no crack exist, one must be produced by applying the point of the heated rod to any convenient spot on the edge of the broken glass, touching it afterwards with a moistened finger, if necessary. The edges of glass thus divided are rendered less apt to break by heating them in the flame of a blowpipe, or grinding them smooth with emery on a flat stone. See Faraday's Manipulations. Glass, to Platinize. M. Dullo. This is recommended to prevent fusing of the end of the tube used in Marsh's test for arsenic. In drawing out the end of the tube, leave the diameter slightly larger than is necessary; then roughen the narrow end with a file. Dip in a solution of bichloride of platinum, containing one twentieth of the metal; remove excess of the drop, and heat cautiously till the glass acquires a metallic appearance. Repeat this four or five times. Glass, to Silver. 1. The term silvering is commonly applied to the process of coating the surface of glass with amalgamated tinfoil, in forming mirrors. The tinfoil is rubbed over with quicksilver, and more of the latter poured over it: the plate of glass, perfectly clean and dry, is then applied to it in such a way as to exclude all air-bubbles, and to bring the glass and tinfoil into perfect contact. The plate, after being inclined, so as to allow the superfluous quicksilver to drain off, is loaded with weights, under which it remains till the adhesion is complete. To convex and concave mirrors, the amalgamated foil is applied by means of accurately fitting plaster moulds. The interior of globes is silvered by introducing a liquid amalgam (see Amalgams), and turning about the globe, till every part is covered with it.