Secret Correspondence Using Invisible Ink

1900

Inks Composed Mainly of Salts and Cobalt Are the Best

Of the many invisible and sympathetic inks that have been used for secret correspondence perhaps the best known are those composed mainly of salts of cobalt. Marks made with these fluids remained invisible until they were subjected to heat, and then were revealed in lines of pale green. The phenomenon was transient and entirely dependent on the temperature. As soon as the sheet of paper grew cool writing would disappear.

Now, to those who had occasion to employ such means of communication it was desirable to know whether any one tampered with their letters or not. So long as the fact of the correspondence was kept secret, of course, there would be no danger. But if that once became known the contents of a private letter might be ascertained by anyone through whose hands the letter passed. Cobalt writing can be made to appear and disappear a dozen times without giving the least evidence of having been read.

But a patent was recently granted in Germany which meets this difficulty, and in some other respects, also, is an improvement on the old system. In the first place the paper used is soaked in the cobalt solution and is prepared in advance. The inventor aims to put his stationery on the market. The writing is done with a solution of common salt and behaves as the cobalt ink did formerly. It can be seen only when warmed and disappears immediately on cooling. Moreover, it appears as often as heat is applied. Its color is a bluish green.

But the German also provides what he calls a "control ink." This may be prepared by adding two grains of resorcin to eight drops of water and six drops of sulphuric acid. When a person has written his letter with salt water he makes a few supplementary remarks, in a spot previously agreed upon, with the control ink. So long as the letter remains cool these test marks are invisible, but when the heat is applied they come out and they come out to stay. They are of a brown hue, different from that of the salt writing, and they will not disappear when the green writing does. If the authorized recipient of a letter finds these marks on a letter before he himself warms it he has reason to suspect that his secret is known. But if they are missing the opposite conclusion is justified.