The Manufacturer and Builder 8, 1877
Nothing is easier than to send secret messages on postal cards, by using what is called "sympathetic ink," which makes an invisible writing until developed. Chemistry teaches a multitude of such inks; for instance, write with a weak solution of bichromate of potash. If the receiver of the postal-card dips it into a decoction of logwood, or a solution of logwood extract, the letters will at once become black. If written with a solution of the yellow ferro-cyanid of potassaid, it has only to be washed over with a solution of sulphate of iron to make blue letters, or sulphate of copper to make reddish-brown letters. When written with a solution of acetate of lead or lead-sugar, the receiver has only to expose the card to sulphurous vapors, such as are developed when sulphuret of iron is moistened with diluted sulphuric acid, to cause black letters to appear. If written with a soluble salt of arsenic, the same sulphurous vapors will develop yellow letters; and if written with a soluble salt of antimony, it will cause orange-red letters to appear. If written with a solution of nitrate or chlorid of nickel, the receiver has only to warm the card before a fire or stove, when green letters will appear; if nitrate or chlorid of cobalt is used, red letters will appear; with chlorid' of copper, bluish letters. It is well to put into these solutions a very little gum or sugar; the beauty of the latter substances is that by cooling the writing disappears again. Even milk alone can be used for writing, which becomes visible on being warmed; this is perhaps the simplest method, but not qaite so satisfactory in its results.