SYMPATHETIC INK is an old invention. Among the methods by which Ovid teaches young women to deceive their guardians, when they write to their lovers, he mentions that of writing with new milk, and of making the writing legible by coal-dust or soot.
Tuta quoque est, fallitque oculos, e lacte recenti
Litera: carbonis pulvere tange, leges.
It is obvious, that any other colourless and glutinous juice, which will hold fast the black powder strewed over it, will answer the purpose as well as milk; and therefore Pliny recommends the milky juice of certain plants to be used.
There are several metallic solutions perfectly colourless, or, at least, without any strong tint, which being wrote with, the letters will not appear until the paper be washed over with another colourless solution, or exposed to the vapour of it; but among all these there is none which excites more astonishment, or from which naturalists can draw more conclusions, than that which consists of a solution of lead in vegetable acid, and which by the vapour of arsenical liver of sulphur becomes black, even at a considerable distance. This ink, which may
may be used by conjurors, proves the subtlety of vapour, and the porosity of bodies; as the change or colouring takes place even when the writing is placed on the other side of a thin wall.
We knew before, that a solution of lead, treated in this manner, would answer the purpose of a sympathetic ink (see that article Encycl.); but we did not know, nor do we yet believe, that the sulphuric vapours will act upon the writing through a wall. Such, however, is the affirmation of Professor Beckmann, who gives an account of a still more wonderful ink from Peter Borel. This author, in a book called Historiarum et observationum medico-physic. centuria quatuor, printed at Paris, first in 1653, and afterwards in 1657, gives a receipt for making this ink, which he calls magnetic waters which act at a distance. The receipt is as follows:
"Let quick lime be quenched in common water, and while quenching, let some orpiment be added to it (this, however, ought to be done by placing warm ashes under it for a whole day), and let the liquor be filtered, and preserved in a glass bottle well corked. Then boil litharge of gold, well pounded, for half an hour with vinegar, in a brass vessel, and filter the whole through paper, and preserve it also in a bottle closely corked. If you write any thing with this last water, with a clean pen, the writing will be invisible when dry; but if it be washed over with the first water it will become instantly black. In this, however, there is nothing astonishing; but this is wonderful, that though sheets of paper without numbers, and even a board, be placed between the invisible writing and the second liquid, it will have the same effect, and turn the writing black, penetrating the wood and paper without leaving any traces of its action, which is certainly surprising; but a fetid smell, occasioned by the mutual action of the liquids, deters many from making the experiment. I am, however, of opinion, that I could improve this secret by a more refined chemical preparation, so as that it should perform its effect through a wall. This secret (says Borel) I received, in exchange for others, from J. Brofson, a learned and ingenious apothecary of Montpellier."
For making a sympathetic ink of the fifth class mentioned in the Encyclopedia, the following process by M. Meyer may be worthy of the reader's notice. It was entered upon in consequence of a receipt for rose-coloured sympathetic ink shewn to him by a traveller. In that receipt cobalt was the principal ingredient, and therefore the first object was to procure cobalt; but M. Meyer, being unwilling to sacrifice pure pieces of cobalt of any considerable size, made choice of one, which was visibly mixed with bismuth, iron, and quartz. He endeavoured to separate the bismuth as much as possible, and also the arsenic, if it should contain any, by bringing it slowly to a red heat; and he succeeded pretty well, as the bismuth flowed from it in abundance; and the arsenic, the quantity of which was small, was volatilised: many globules of bismuth still adhered to it. By bringing it repeatedly to a red heat, and then quenching it in water, it was reduced to such a state as to be easily pulverised. Having poured nitrous acid upon the powder, he obtained by digestion a beautiful rose red solution; the siliceous earth was separated in the form of a white slime, and by diluting it with water there was deposited a white powder, which was oxyd of bismuth. The solution being filtered, he added to it a solution of
potash, and obtained a precipitate inclining more to a yellow than to a red colour. He again poured over it a little of the nitrous acid, by which a part of the oxyd was re-dissolved of a red colour: the remaining part, which had a dark brown colour, was oxyd of iron. From the solution, by the addition of potash, a precipitate was formed, which was now reddish. Having by this process obtained it pure, that he might now prepare from it the wished-for red ink, he dissolved the washed pure oxyd of cobalt in different acids. That dissolved in the nitrous acid with a mixture of nitre, gave a green ink like the common: that dissolved in the sulphuric acid, without the addition of salts, gave a reddish ink, which remained after it was exposed to heat, and would not again disappear, even when a solution of nitre was applied; and that dissolved in the muriatic acid, gave a green ink, darker and more beautiful than the common. By dissolving it, however, in the acetic acid, and adding a little nitre, he obtained what he had in view; for it gave, on the application of heat, an ink of a red colour, like that of the rosa centifolia, which again disappeared when the paper became cold.