9. Digest 1 oz. of zaffre, or cobalt oxide, at a gentle heat, with 4 oz. of nitro-muriatic acid till no more is dissolved, then add 1 oz. common salt and 16 oz. of water. If this be written with and the paper held to the fire, the writing becomes green, unless the cobalt should be quite pure, in which case it will be blue. The addition of a little iron nitrate will then impart the property of becoming green. It is used in chemical landscapes for the foliage. 10. Put in a vial ½ oz. of distilled water, 1 drm. of potassium bromide and 1 drm. of pure copper sulphate. The solution is nearly colorless, but becomes brown when heated. 11. Nickel nitrate and nickel chloride in weak solution form an invisible ink, which becomes green by heating when the salt contains traces of cobalt, which usually is the case; when pure, it becomes yellow. 12. When the solution of acetate of protoxide of cobalt contains nickel or iron, the writing made by it will become green when heated; when it is pure and free from these metals, it becomes blue. 13. Milk makes a good invisible ink, and buttermilk answers the purpose better. It will not show if written with a clean new pen, and ironing with a hot flat iron is the best way of showing it up. All invisible inks will show on glazed paper; therefore unglazed paper should be used. 14. Burn flax so that it may be rather smoldered than burned to ashes, then grind it with a muller on a stone, putting a little alcohol to it, then mix it with a little gum water, and what you write, though it seem clear, may be rubbed or washed out. 15. Boil cobalt oxide in acetic acid. If a little common salt be added, the writing becomes green when heated, but with potassium nitrate it becomes a pale rose color. 16. A weak solution of mercury nitrate becomes black by heat. _Inks that Appear under the Influence of Light._ 17. Gold chloride serves for forming characters that appear only as long as the paper is exposed to daylight, say for an hour at least. 18. Write with a solution made by dissolving one part of silver nitrate in 1,000 parts of distilled water. When submitted to daylight, the writing appears of a slate color or tawny brown. _Inks Appearing through Reagents._ 19. If writing be done with a solution of lead acetate in distilled water, the characters will appear in black upon passing a solution of an alkaline sulphide over the paper. 20. Characters written with a very weak solution of gold chloride will become dark brown upon passing a solution of tin perchloride over them. 21. Characters written with a solution of gallic acid in water will become black through a solution of iron sulphate and brown through the alkalies.