20. SYMPATHETIC INKS FOR SECRET CORRESPONDENCE.--_Process 1._--Dissolve muriate of ammonia in water, and write;--the writing will be invisible. When you would make the writing appear, heat the paper by the fire, and the writing will become black. 21. _Process 2._--Write with a solution of sulphate of iron--the writing will be invisible. Dip a feather in an infusion of nut-galls, and with it wet the paper, and the writing will become black. 22. _Process 3._--Write with a dilute infusion of galls,--it will be invisible. Dip a feather in a solution of sulphate of iron, and moisten the paper with it and the writing will become black. 23. _Process 4._--Write with a solution of sub-carbonate of potass; wet this writing with a solution of sulphate of iron,--it will take a deep yellow colour. 24. _Process 5._--Write with a solution of sulphate of copper,--no writing will be visible. Wash the paper with a solution of prussiate of potass,--the writing will then get a reddish brown colour. 25. _Process 6._--Write with a solution of super-carbonate of soda;--moisten the paper with a solution of sulphate of copper, and the writing will become green. 26. _Process 7._--Write with diluted nitrate of silver, and let the writing dry in the dark--it will be invisible; but expose the paper to the rays of the sun, and the writing will become black. 27. LUMINOUS INK THAT WILL SHINE IN THE DARK.--To half an ounce of essential oil of cinnamon, in a phial, add half a drachm of phosphorus. Cork the phial slightly, and set it, or suspend it near a fire, where the heat may be nearly equal to boiling; continue the heat four or five hours, shaking the phial frequently, but cautiously lest any of the oil should escape, or come in contact with atmospheric air, in which case it would take fire. The cork should be set sufficiently tight to exclude atmospheric air, but not so as to prevent the escape of any vapour that might be produced by excess of heat. The phial may be afterward removed from the fire and suffered to cool. With this phosphorised oil, any letters may be written on paper, and if carried into a dark room, will appear very bright, resembling fire. The phial should be kept corked close, except when used. 28. TO MAKE A WRITING APPEAR AND DISAPPEAR AT PLEASURE.--Dissolve equal parts of sulphate of copper and muriate of ammonia in water, and write. When you would make the writing appear, warm the paper gently by the fire; the writing will appear in a yellow colour; but as soon as you take the paper into the cold air, the writing will vanish. This may be often repeated.