METALS.

287

MRS. B.

It is produced by the intervention of oxygen ;for verdigris is a compound salt formed by theunion of vinegar and copper; it is of a beautifulgreen colour, and much used in painting.

EMILY.

But, I believe, verdigris is often formed on cop-per when no vinegar has been in contact with it.

MRS. n.

Not real verdigris, but compound salts, some-what resembling it, may be produced by the ac-tion of any acid on copper.

There is a beautiful green salt produced by thecombination of cobalt with muriatic acid, whichhas the singular property of forming what iscalled sympathetic ink. Characters written withthis solution are invisible when cold, but when agentle heat is applied, they assume a fine blueishgreen colour.

CAROLINE.

I think one might draw very curious land-scapes with the assistance of this ink; l wouldfirst make a water-colour drawing of a winterscene, in which the trees should be leafless, andthe grass scarcely green ; l would then trace allthe verdure with the invisible ink, and whenever1 chose to create spring, I should hold it before