Young's Demonstrative Translation of Scientific Secrets / Or, A Collection of Above 500 Useful Receipts on a Variety of Subjects

by Daniel Young

47. SECRET INK FOR YOUNG LADIES AND GENTS

Take a drachm of clean rain water, put into it, in a clean vial, 10 or 12 drops of pure, clean sulphuric acid, and it is ready for use; write with this using a clean quill pen on letter paper, and when dry you can see no mark at all, then hold it to a strong heat and the writing becomes as black as jet. If you want to write to a young lady or gentleman, as the case may be, and fearing that the letter might be opened before she or he gets it, write with common black ink something of no importance, then between the lines write what you want to say with the secret ink. The person to whom you are writing must understand the scheme so that she or he may hold it to the heat and thereby make the writing visible.