The Romance of Modern Chemistry

ACIDS AND ALKALIS remember that it contains a fair quantity of acetic acid, which, although not so powerful as sulphuric or hydro- chloric acid, is yet able to exhibit the corrosive action which is characteristic of acids in general. A drop of vinegar left on the surface of a copper saucepan will betray itself before long by the appearance of verdigris; this is acetate of copper, the salt produced by the action of the acetic acid in the vinegar on the copper of the saucepan. The wit of man has hit upon methods of utilising this corrosive action of acids on metals, and if properly guided it becomes a process of engraving. Suppose we have a plate of copper on which we wish to trace some design. One simple way of doing this is to coat the plate com- pletely with a thin film of wax or other substance which is not affected by acids, and then with a sharp steel point to scratch the required design through the wax. This means that the metal surface is exposed where the steel point has removed the wax, so that if the plate is im- mersed in an acid bath, say of “aqua fortis” (nitric acid), the metal is eaten away along the lines traced by the engraver. When the wax is dissolved, the metal plate is then found to bear the design intended, the depth of the lines depending on the length of time during which the plate has been left in the bath. Such etching of metals is characteristic of acids gener- ally, but there is another kind of etching, namely, on glass, which can be effected by one acid only—that is to say, it is a specific action. This very peculiar property is possessed by hydrofluoric acid, a compound of hydrogen and the element fluorine, which is combined with lime in the mineral fluorspar. If a piece of glass is coated with paraffin wax, and a design is traced upon this with a 83