For the silver, use the same proportion of metal as for the gold tree.
Sympathetic or Secret Inks.
Sympathetic or secret inks are those fluids which, when written with on paper, are invisible when dry, but acquire colour by heating the paper, or by applying to the invisible writing another chymical agent. These phenomena arrested particularly the attention of the old chymists, and accordingly, in their fanciful way, they called them sympathetic inks.
The writing made with this ink may, therefore, at pleasure be made to become visible or invisible successively, by alternately warming and cooling, if care be taken not to expose the paper to a greater degree of heat than is necessary to make the invisible writing legible.
Preparation of Green Sympathetic Ink.
Put into a mattrass one part of cobalt or zaffree, and four ounces of nitro-muriatic acid. Digest the mixture with a gentle heat until the acid dissolves no more; then add muriatic of soda, equal in quantity to the cobalt employed, and four times as much water as acid, and filter the liquor through paper.
Blue Sympathetic Ink.
This ink, which may be used like the preceding, is prepared in the following manner. Take one ounce of cobalt reduced to powder, put it into a Florence flask, and pour over it two ounces of pure nitric acid. Expose the mixture to a gentle heat, and, when the cobalt is dissolved, add, by small quantities, a solution of potash, until no more precipitate ensues. Let these precipitates subside: decant the super-natant fluid, and wash the residuum repeatedly in distilled water, until it passes tasteless; then dissolve it in a sufficient quantity of distilled vinegar, by the assistance of a gentle heat, taking care to have a saturated solution, which will be known by part of the precipitate remaining undissolved after the vinegar has been digested on it for some time.
Yellow Sympathetic Ink.
Neutralize muriatic acid with brown oxide of copper: the solution is of a dark olive-green colour, and by evaporation yields crystals of a grass-green colour of muriate of copper, which, when dissolved in ten parts of water, forms this ink, and may be employed as before stated.