The hardest glass may be etched and frosted with a peculiar liquid acid, and also with this acid in the condition of vapor. When powdered fluor spar is heated with concentrated sulphuric acid in a platinum or a lead retort, and connected with a refrigerator by a tube of lead, a very volatile, colorless liquid is obtained, which emits copious white and suffocating fumes. This is hydrofluoric acid, a dilute solution of which attacks glass with avidity, while neither sulphuric, nitric, nor muriatic acid has the least effect upon it. In a diluted state it is employed for glass etching, for which purpose it is kept in a lead vessel, because it has very little affinity for this metal. The vapor of this acid is also used for the same purpose. The glass to be operated upon is first coated with a ground of wax, and the design to be etched is then traced through the wax with a sharp instrument. In a shallow lead basin some powdered fluor spar is then placed, and a sufficient quantity of sulphuric acid poured upon it to convert it into a thin paste. The glass to be etched is now placed in the basin, to which a gentle heat is applied, when the vapor of the acid is disengaged and attacks the traced lines from which the wax has been removed. The operation is completed in a few minutes, the glass is removed, and the wax cleaned off with warm oil of turpentine. All those parts which have remained covered with the wax are now clear as before, while the other parts drawn by lines to represent figures have a frosted appearance. Any person can produce figures on glass with this acid, but, for reasons before stated, it is dangerous to use. In October, 1859, a patent was granted to James Napier, of Glasgow, Scotland, for a very simple method of ornamenting glass with fluoric acid. Instead of drawing patterns and figures on the glass with the use of varnish and a graver to prepare the glass for etching, the glass is prepared by simply transferring pictures from prints, which can be performed by almost any person. The method is, to take a print, lithograph, or picture made with printer's ink, and fix the printed surface to the glass by any ordinary paste made from starch. All the air must be carefully excluded from between the print and glass. When perfectly dry, liquid hydrofluoric acid about the specific gravity of 1.14 is applied for about three minutes, when it is washed in water to remove the paper and the acid, and the figure of the print is then found upon the glass. The printed portion of the paper may also be cut in outline and pasted on the glass, then transferred. Glass that is "flashed" on the surface with another color may be treated in this manner, when a portion of the flashing or surface will be removed, and the picture will remain in color. COLORED GLASS.