226 SCIENTIFIC AMUSEMENTS. 



soluble in water. To prepare the gas powdered fluor spar (fluoride 

 of calcium) is heated with oil of vitriol, when a chemical change 

 occurs somewhat resembling that taking place in the last experi- 

 ment, hydrofluoric acid and sulphate of calcium, being formed by 

 double decomposition. A leaden trough is used, made by bending 

 up the corner of a piece of sheet lead some 6 or 8 inches square, 

 so that the tray produced is about an inch or 1J inches deep; 

 powdered fluor spar is strewed over the tray to the depth 

 of the thickness of a penny piece, and enough oil of vitriol 

 poured over the powder to wet it thoroughly and make a thin 

 paste. The tray is then placed on an iron tripod and covered over 

 with the plate of glass prepared for etching, and gently warmed 

 with a lamp to bring about the action of the acid more quickly. 

 The glass is prepared by carefully warming it, then dropping on 

 melted beeswax, and causing this to run over the entire surface 

 and allowing it to solidify, so as to form a coating on the glass ; 

 when this has set the design to be etched is cut through the wax 

 by means of the point of a needle or penknife or other convenient 

 tool, so as to lay bare the glass where the cuts are made ; the 

 design may be drawn on tissue, paper with a pencil and then trans- 

 ferred to the wax by pressing on the paper with a uniform gentle 

 pressure and then peeling it off again ; if the pencil be of good 

 quality the marks will be more or less completely retained on the 

 surface of the wax. The glass is then set on the leaden trough 

 lid-fashion with the prepared side downwards, so that any vapour 

 evolved in the tray will come in contact with the glass where it is 

 exposed by cutting away the protecting wax ; at these points the 

 glass is attacked and corroded, but not at the other portions of its 

 surface covered with wax. When the action has gone on long 

 enough, the glass is removed and warmed and the wax wiped off. 

 Care must be taken not to heat the tray so much as to melt the 

 wax prematurely, and to hide away parts of the etching by the 

 flow of melted wax over the exposed surface of the glass. 



Caution. This experiment should, if possible, be done in a 

 draught place (Expt. 145), or at least in a room well ventilated, 

 and great care must be taken not to breathe in the hydrofluoric 

 acid vapours, as they are even more injurious to the lungs than 

 chlorine or bromine. 



Expt. 272. To engrave on Copper or Steel. Engravings on 

 iron and steel are often executed in a fashion very similar to the 

 above; the metal plate is covered with a coating of "resist" (a 

 substance of resinous and waxy character not affected by the acid 

 subsequently used), and the design traced by cutting away the 

 composition, so as to lay bare the metal ; aqua fortis, or diluted 



