29. TO MAKE A WRITING VANISH AND ANOTHER APPEAR IN ITS PLACE.--Write on paper with a solution of sub-carbonate of potass,--the writing will be invisible. Mix together equal parts of solution of sulphate of iron, and infusion of galls; write with this mixture (which is black) on the same paper. Then add to the black liquor a little sulphuric acid, sufficient to deprive it of colour. Wet the paper with this compound; the acid will discharge the colour from the last writing, while the alkali of the first, will precipitate the gallate of iron, and the writing will become black. 30. TO RESTORE OLD WRITING THAT IS NEARLY DEFACED.--Boil one ounce of powdered nut-galls, for an hour or more in a pint of white wine; filter the liquor, and when cold, wet the paper with it, or pass it on the lines with a camel hair pencil, and the writing will be much revived. 31. TO PAINT A PICTURE THAT WILL APPEAR AND DISAPPEAR OCCASIONALLY.--To half an ounce of nitric acid, add one drachm of cobalt, one drachm of muriate of soda, and two ounces of water; set it in a sand bath or on warm ashes, where it must remain five or six hours. Then filter the solution, (which is nitro-muriate of cobalt,) and with it draw the trees, and shrubbery of a designed picture. Then with a solution of oxide of cobalt in acetic acid, draw some distant mountains, fences, &c. and with muriate of copper, (the compound solution described at 28,) draw some flowers, buildings, &c. These will all be invisible when dry; but warm the paper and the picture will appear in green, blue and yellow. It will disappear again when the paper becomes cold.