Invisible ink

Sympathetic Ink

If we give this name to any fluid, which when written with, will remain invisible till after a certain operation, such liquids were known in very early periods. Among the methods, with which Ovid teaches young women to deceive their guardians, when they write to their lovers 260 , he mentions that of writing with new milk, and of making the writing legible by coal-dust or soot. Ausonius proposes the same means to Paulinus 261 ; but hiscommentators seem not to have fully understood his meaning; for favilla  is not to be explained by favilla non modice calida, as Vinetus has explained it, but by fuligo. That word is often employed by the poets in the same sense. As a proof of it, Columella, speaking of the method, not altogether ineffectual, and even still used, of preserving plants from insects by soot, calls it nigra favilla ; and afterwards, when mentioning the same method, free from poetical fetters, he says fuliginem quæ supra focos tectis inhæret 262 . It may be easily perceived, that instead of milk any other colourless and glutinous juice might be employed, as it would equally hold fast the black powder strewed over it. Pliny, therefore, recommends the milky sap of certain plants for the like purpose 263.

There are several metallic solutions perfectly colourless, or, at least, without any strong tint, which being used for writing, 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, than that which consists of a solution of lead in acetic acid, and which by sulphuretted hydrogen gas becomes black, even at a considerable distance. This ink, which may be employed by conjurers, proves the subtlety of this gas, 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.

This effect presented itself perhaps accidentally to some chemist; but the discovery is not of great antiquity. Wecker, who compiled his book De Secretis from Porta, Cardan, and several old writers, and printed it for the first time in 1582, and gave a third edition in 1592, must have been unacquainted with it; else he certainly would not have omitted it in the fourteenth book, where he mentions all the methods of secret writing. Neither would it have been unnoticed by Caneparius, whose book De Atramentis was printed at Venice, for the first time, in 1619.

The first person who, as far as I have been able to learn, gave a receipt for preparing this ink, was Peter Borel, in Historiarum et Observationum Medico-physic. Centuriæ quatuor. In this work, which was printed for the first time in 1653, and a second time in 1657, at Paris, and of which there were several editions afterwards, the author calls it a magnetic water, which acts at a distance 264. After the occult qualities of the schoolmen were exploded, it was customary to ascribe phænomena, the causes of which were unknown, and particularly those the causes of which seemed to operate without any visible agency, to magnetic effluvia; as the tourmaline was at first considered to be a kind of magnet. Others concealed their ignorance under what they called sympathy, and in latter times attraction and electricity have been employed for the like purpose. Borel, who made it his business to collect new observations that were kept secret, learned the method of preparing this magnetic water from an ingenious apothecary of Montpelier, and in return taught him some other secrets. Otto Tachen, a German chemist 265 , afterwards thought of the same experiment, which he explains much better, without the assistance of magnetism or sympathy. The receipt for making these liquids, under the name of sympathetic ink, I find first given by Le Mort 266 , and that name has been still retained 267.

Another remarkable kind of sympathetic ink is that prepared from cobalt, the writing of which disappears in the cold, but appears again of a beautiful green colour, as often as one chooses, after being exposed to a moderate degree of heat.

The invention of this ink is generally ascribed to a Frenchman named Hellot. He was, indeed, the first person who, after trying experiments with it, made it publicly known, but he was not the inventor; and he himself acknowledges that a German artist of Stolberg first showed him a reddish salt, which, when exposed to heat, became blue, and which he assured him was made out of Schneeberg cobalt, with aqua regia 268. This account induced Hellot to prepare salts and ink from various minerals impregnated with cobalt; but A. Gesner proved, long after, that this ink is produced by cobalt only, and not by marcasite 269.

When Hellot's experiments were made known in Germany, it was affirmed that Professor H. F. Teichmeyer, at Jena, had prepared the same ink six years before, and shown it to his scholars, in the course of his lectures, under the name of sympathetic ink 270. It appears, however, that it was invented, even before Teichmeyer, in the beginning of the last century by a German lady. This is confirmed by Pot, who says that the authoress of a book printed in 1705, which he quotes under the unintelligible title of D. J. W. in clave, had given a proper receipt for preparing the above-mentioned red salt, and the ink produced by it 271. If it be true that Theophrastus Paracelsus, by means of this invention, could represent a garden in winter, it must be undoubtedly older 272.

[In consequence of the progress of modern chemistry and the discovery of a vast number of new chemical compounds, sympathetic inks may be made in an almost endless number and variety. The principal may be classed in the following manner:—1, such as when dried upon paper being invisible, on moistening with another liquid become again evident : of this kind there are a vast number; among which we may mention a solution of a soluble salt of lead, or bismuth, for writing, and a solution of sulphuretted hydrogen for washing over; the writing then appears black; or green vitriol for writing and prussiate of potash for washing over, when the writing becomes blue 273 ; 2, such as are rendered evident by being sifted over with some powder, as the milk with soot described above; 3, those which become visible by heat, such as characters in dilute sulphuric acid, lemon-juice, solutions of the nitrate and chloride of cobalt, and of chloride of copper; the two former become black or brown, the latter are rendered green, the colouring disappearing subsequently when allowed to cool in a moist place. Amusing pictures are sometimes made with these sympathetic inks, particularly those composed of cobalt; for if a landscape be drawn to represent winter, the vegetation being covered with a solution of cobalt, on holding the paper to the fire, all those portions covered with the solution appear of a bright green, and thus completely change the character of the scene.]

Footnotes

260  De Arte Amandi, lib. iii. v. 629.

261  Ausonii Epist. xxiii. v. 21. The poet afterwards teaches other methods of secret writing, and Gellius, lib. xvii. cap. 9, mentions the like.

262  Colum. De Re Rust. x. 354. and xi. 3, 60.

263  Plin. lib. xxvi. cap. 8. p. 400.

264  The sixth observation of the second century is as follows: Magnetic waters which act at a distance. An astonishing effect, indeed, is produced by the contest of the following waters, which are thus made. 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 number, 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 that it should perform its effect through a wall. This secret I received, in exchange for others, from J. Brosson, a learned and ingenious apothecary of Montpelier.

265  Tachenii Hippocraticæ Medicinæ Clavis, p. 236. 1669.

266  Collectanea Chymica Leydensia, edidit Morley. Lugd. Bat. 1684, 4to, p. 97.

267  For an account of various kinds of secret writing see Halle, Magie oder Zauberkräfte der Natur. Berlin, 1783, 8vo, v. i. p. 138.

268  Hist. et Mém. de l'Acad. des Sciences à Paris, 1737, pp. 101 and 228.

269  Historia Cadmiæ fossilis, sive Cobalti. Berl. 1744.

270  This account, together with Teichmeyer's receipt for preparing it, may be found in Commercium Litterarium Norimbergense, 1737, p. 91.

271  “Copiosius minera bismuthi tam ab aqua forti quam ab aqua regia dissolvitur, restante pulvere albo corroso; solutio in aqua forti roseum colorem sistit, quæ si sali in aqua soluto, secundum præscriptum D. J. W. in clave, affundatur, abstrahatur, ex residuo extrahitur sal roseum, quod pulverisari et cum spiritu vini extrahi potest: adeoque hæc autrix jam anno 1705 publice totum processum et fundamentum sic dicti atramenti sympathetici, quod a calore viridescit, evulgavit.”—Pot, Observ. Chym. collectio prima. Berolini, 1739, p. 163.

272  So thinks Gesner in Selecta Physico-œconomica, or Sammlung von allerhand zur Naturgeschichte gehörigen Begebenheiten. Stutgard, vii. p. 22.

273

[Inks formed of solutions
of the following salts
,
and washed with
solutions of
become
Muriate of antimony,tincture of galls,yellow.
Green vitriol,tincture of galls,black.
Nitrate of cobalt,oxalic acid,blue.
Subacetate of lead,hydriodic acid,yellow.
Arseniate of potash,nitrate of copper,green.
Nitrate of copper,prussiate of potash,brown.
Solution of gold,muriate of tin,purple.
Perchloride of mercury,hydrochlorate of tin,black.]