Invisible Ink. The most curious of all kinds of sympathetic ink, is that from cobalt. It is a very singular phenomenon, that the characters or figures traced out with this ink may be made to disappear and reappear at pleasure. This property is peculiar to ink obtained from cobalt, for all the other kinds are at first invisible, until some substance has been applied to make them appear; but when once they have appeared they remain. To prepare this ink, take zaffre, and dissolve it in nitro-muriatic acid, till the acid extracts from it the metallic part of the cobalt, which communicates to the zaffre its blue colour; then dilute the solution, which is very acrid, with common water. If you write with this liquor on paper, the characters will be invisible ; but when exposed to a sufficient degree of heat they will become green. When the paper has cooled they will disappear. Observe, if the paper be too much heated, they will not disappear at all.