Writing done with sympathetic or secret inks is not perceived till the paper has undergone some preparation to render the characters visible. A great number of suitable preparations are known. The following is the composition of a sympathetic ink that becomes visible on being warmed. Form a very weak solution of equal parts of blue vitriol and sal-ammoniac in water. Another, nitrate of nickel and chloride of nickel in weak solution. Another, dissolve 25 gr. of chloride of cobalt in 1 oz. of water. Another, dissolve 1 part bromide of potassium and 1 part blue vitriol in S parts water and 1 part alcohol. Another, dilute sulphuric acid with water. Another, make a weak solution of cobalt in nitro-muriatic acid. Writing made with weak tincture of galls is invisible till wetted with a weak solution of sulphate of iron; vice versa, a weak solution of sulphate of iron is not visible till moistened with a solution of galls. If a solution of alum be' employed, the characters will be invisible till the paper be immersed in water. A solution of acetate of lead in water will not appear till moistened with a solution of sulphuret of potass, which renders it brown. To make a blue sympathetic ink, dissolve cobalt in nitric acid, and precipitate it by potash.

Dissolve this pi'eeipitated oxide of cobalt in acetic acid, and add to the solution one-eighth of common salt; for the writing to appear, heat the paper. It is an awkward matter to write with a colourless fluid, so any of these inks may be mixed with powdered burnt cork. When dry. the blackness may be removed by the use of indiarubber.