The Romance of Modern Chemistry

ACIDS AND ALKALIS sharp-pointed instrument, then on exposure to the vapour of hydrofluoric acid the glass is eaten away at the places where the wax has been removed. In this way the design traced upon the wax is reproduced on the glass, the sur- face, however, being deadened where the acid vapour has acted. If it is desired to etch without deadening the surface, the glass is immersed in a solution of hydrofluoric acid; this treatment leaves the glass polished and clear even where it has been etched. The process of etching on glass is invaluable to the maker of scientific instruments, for it is frequently desir- able to have figures marked on glass apparatus itself rather than on any scale attached to the apparatus. An “ ink ” is actually sold containing the ingredients necessary to produce hydrofluoric acid, and mere writing with this on a piece of glass apparatus is sufficient to leave an im- pression. The constituent of glass attacked by hydrofluoric acid is silica. This is the oxide of the element silicon, and forms a very large proportion of the rocks in the earth’s crust. Sand, for example, is impure silica, and the behaviour of hydrofluoric acid towards silica is well illus- trated by allowing it to act on powdered sand in a leaden vessel. The sand gradually disappears, because the silicon in it forms a gaseous compound with the fluorine con- tained in the hydrofluoric acid. It is difficult to realise that the essential constituent of sand may be converted into a vapour, but that is what is effected by the action of the hydrofluoric acid. Bearing in mind this corrosive action of hydrofluoric acid on glass, we see that it would be inadvisable to keep a solution of the acid in any glass vessel; as a matter of fact, it is usually sold in gutta-percha bottles. 84