Nearly all those experiments which result in the production of color may be performed in another way, and be then applied to the purposes of secret writing. Thus:
Write with dilute solution of sulphate of copper. The writing will be quite invisible, but become blue when held over the vapor of liquid ammonia.
Write with the same solution, and wash the paper with solution of yellow prussiate of potash, and the writing, previously invisible, will become brown. If you choose you may reverse this method, writing with solution of the prussiate of potash, and washing the paper with solution of the copper salt.
Write with solution of sulphate of iron, and the writing will again be invisible. Wash it over with tincture of galls, and it becomes black.
Write with sulphate of iron, and use a wash of yellow prussiate of potash, and the writing will come out blue. This experiment may likewise be reversed, and with similar result.
How to Copper a Knife-Blade.
Make a rather strong solution of sulphate of copper. Let a clean and polished piece of steel or iron, such as the blade of a knife, stand in it for a few minutes, and the iron will become covered or encrusted with a deposit of pure copper.
To Make Beautiful Crystals.
Dissolve, in different vessels, half an ounce each of the sulphates of iron, zinc, copper, soda, alumina, magnesia, and potash. The solutions can be made more rapidly by using warm water. When the salts are all completely dissolved, pour the whole seven solutions into a large dish, stir the mixture with a glass rod, then place it in a warm place, where it will not be disturbed. By degrees, the water will evaporate, and then the salts will re-crystallize, each kind preserving its own proper form and color. Some occur in groups, some as single crystals. If carefully protected from dust, these form extremely pretty ornaments for the parlor.