in a warm air, and while it is cold no colour can be perceived: but gently warming it before the fire, the writing gradually acquires a greenish blue colour, which is visible as long as the paper continues a little warm, and disappears entirely when it cools. When other sympathetic inks are made to appear by proper application, they do not disappear again; but this liquor from the ore of bismuth must have the fire or heat kept to it, to render it legible. If a man writes to his mistress, suppose, or to a minister of state, with lemon juice, once the writing has been warmed by the fire, and the letters by that means appear, the epistle may be afterwards read at any time and place; but if the lady's father should by accident get your letter, written in lilach-coloured liquor, it must still remain a secret to him: For if on getting it, and opening the seal, he could see no writing, and therefore imagining it was writ with lemon juice, or

some other sympathetic ink, he should hold it himself to the fire, or bid his servant hold it to the heat, that the letters might be produced, and made visible, yet the moment bismuth-ink is taken away from the fire, and begins to cool, it is as invisible again, as a sheet of white paper. How serviceable this may be on various occasions, may be easily conceived.
*But as to our third semi-metal, called Zinc, this is so like bismuth to appearance, that some have confounded it with Zinc; though it differs from it essentially in its properties, and will unite with all metalline substances, except bismuth. It is volatile by fire above all things, and makes a sublimate of the metallic substances with which it is fused. Zinc mixed with copper in the quantity of a fourth part, produces brass. If the Zinc is not very pure, the composition proves tombac, or Prince's metal.
*Regulus of arsenic, the fourth semi-metal, has a colour resembling lead, unites readily with metallic substances, and renders them brittle, unmalleable, and volatile. The calx of it produced by fire, may be made volatile by more fire, and in this differs from the calx of all metalline substances; for all other calx's are fixed, and cannot be moved. It has likewise a saline character, in which its corrosive quality or poison consists: a quality from

which the other metallic substances are free, when they are not combined with a saline matter.