An ink that is invisible for some time is a solution of silver nitrate in ammonia, which gradually becomes black when acted upon by air and light. Or certain dye-stuffs such as magenta, that have been treated with a bleaching reagent in just sufficient quantity to decolorise them fulfil the same purpose, the original colour gradually reappearing as the oxygen of the air acts upon the pigment. The earliest inks that were rendered visible by chemical reagents were believed to act by magnetism. Thus in a medical book of the seventeenth century, written by Brossonius, a "magnetic fluid" is described made from "arseniated liver of sulphur," which only became visible when looked at with the "eyes of affection." This appears to have been nothing more mysterious than an ink of lead acetate, the characters written with which could be rendered visible by exposing them to the vapour of sulphuretted hydrogen. Inks of this kind were also mentioned, in 1669, by Otto Tachen, who referred to them as _aquæ magnetice e longinquo agentes_, but pointed out that there was nothing magnetic in their action. The term _sympathetic ink_ appears to have originated with Le Mort, who applied it to the lead acetate ink, and later on the name was extended to all secret inks. The best known sympathetic inks consist of solutions of cobalt salts, the writing done with which changes on heating from a nearly invisible pink to blue. This peculiarity of cobalt to form two series of salts containing different amounts of water was discovered in 1715 by Waiz. Other compounds that may be used as sympathetic writing fluids include tannin, which forms ordinary ink on the addition of iron sulphate; cobalt nitrate which becomes blue on adding oxalic acid, and gold chloride which gives a purple colour with tin chloride. Some thirty years ago a patent was taken out by Kromer for the use of a sympathetic ink in detecting any tampering with envelopes. The two dried constituents of the ink, say tannin and iron sulphate, are separated by the adhesive gum upon the envelope, so that should steam be applied to open the letter, the two substances come into contact, and form an ink, which leaves a stain upon the paper. Printing inks based upon these principles are used in preparing the groundwork of cheques, so that any attempt to remove the writing from the cheque by means of chemical agents will be betrayed by the change of colour upon the body of the paper. The value of sympathetic inks in detecting an offender was strikingly shown in the recent Sutton libel case, in which a woman was found guilty of sending offensive cards through the post. The story is a very remarkable one. For many months during 1908 and the early part of 1909, there was an epidemic of anonymous post cards in Sutton, many people receiving them and no one being able to trace their origin.