Sympathetic Inks.
Under the name of sympathetic inks are designated certain liquids which, being used for writing, leave no visible traces on the paper, but which, through the agency of heat, or by the action of chemicals, are made to appear in various colors. The use of such means for secret correspondence is very ancient. Ovid, Pliny, and other Roman writers speak of an ink of this kind, which, however, was nothing more than fresh milk. It merely sufficed to dust powdered charcoal over the surface of the paper upon which characters had been traced with the colorless fluid, when the black powder adhered only to those places where the fatty matter of the milk had spread. Such a process, however, was merely mechanical, and the results very crude.
A great number of sympathetic inks may be obtained by means of reactions known to chemistry. For instance, write on paper with a colorless solution of sugar of lead; if the water that is used for the solution be pure, no trace of the writing will remain when it becomes dry. Now hold the paper over a jet of sulphureted hydrogen, and the characters will immediately appear on the paper, of an intense black color. The following recipes for inks of this kind are more simple: If writing be executed with a dilute solution of sulphate of iron, the invisible characters will appear of a beautiful blue, if the dry paper be brushed over with a pencil full of a solution of yellow prussiate of potash; or they will be black, if a solution of tannin be substituted for the prussiate. If the characters be written with a solution of sulphate of copper, they will at once turn blue on exposing to the vapors of ammonia. Another sympathetic ink is afforded by chloride of gold, which becomes of a reddish purple when acted upon by a salt of tin. A red sympathetic ink may be made in the following manner: Write with a very dilute solution of perchloride of iron—so dilute, indeed, that the writing will be invisible when dry. By holding the paper in the vapor arising from a long-necked glass flask containing sulphuric acid and a few drops of a solution of sulpho-cyanide of potassium, the characters will appear of a blood-red color, which will again disappear on submitting them to the vapors of caustic ammonia. This experiment can be repeated ad infinitum.
During the war in India, some years ago, important correspondence was carried on by the English by means of the use of rice water as a writing fluid. On the application of iodine the dispatches immediately appeared in blue characters.
Sympathetic inks which are developed under the influence of heat only are much easier to use than the foregoing. The liquids which possess such a property are very numerous. Almost every one perhaps knows that if writing be executed on paper with a clean quill pen dipped in onion or turnip juice, it becomes absolutely invisible when dry; and that when the paper is heated the writing at once makes its appearance in characters of a brown color. All albuminoid, mucilaginous, and saccharine vegetable juices make excellent sympathetic inks; we may cite, as among the best, the juices of lemon, orange, apple, and pear. A dilute solution of chloride of copper used for writing is invisible until the paper is heated, when the letters are seen of a beautiful yellow, disappearing again when the heat that developed them is removed. The salts of cobalt, as the acetate, nitrate, sulphate, and chloride, possess a like property. When a dilute solution of these salts is used as an ink, the writing, although invisible when dry, becomes blue when exposed to heat. The addition of chloride of iron, or of a salt of nickel, renders them green, and this opens the way for a very pretty experiment: If a winter landscape be drawn in India ink, and the sky be painted with a wash of cobalt alone, and the branches of the trees be clothed with leaves executed with a mixture of cobalt and nickel, and the snow-clad earth be washed over with the same mixture, a magic transformation at once takes place on the application of heat, the winter landscape changing to a summer scene.
There is a well known proprietary article sold in Paris under the name of "Encre pour les Dames" (ink for ladies). Hager, in a recent scientific journal, states that this consists of an aqueous solution of iodide of starch, and is "specially intended for love letters." In four weeks characters written with it disappear, preventing all abuse of letters, and doing away with all documentary evidence of any kind in the hands of the recipient. The signers of bills of exchange who use this ink are of course freed from all obligations in the same length of time.
