The solutions used should be so nearly colourless that the writing cannot be seen till the agent is applied to render it visible.

1. Digest 1 oz. of zaffre, or oxide of cobalt, at a gentle heat, with 4 oz. of nitro-hydrochloric acid till no more is dissolved, then add 1 oz. of 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 nitrate of iron will then impart the property of becoming green. It is used in chemical landscapes, for the foliage.

2. Put into a phial 1/2 oz. of distilled water, 1 dr. of bromide of potassium, and 1 dr. of pure sulphate of copper. The solution is nearly colourless, but becomes brown when heated.

3. Boil oxide of cobalt in acetic acid. If a little common salt be added, the writing becomes green when heated; but with nitre it becomes a pale rose-colour.

4. A solution of acetate of lead. Colourless, but becomes brown when exposed to sulphuretted hydrogen gas.

5. A weak solution of sulphate of copper. The writing becomes blue when exposed to the vapour of ammonia.

6. A solution of sulphate - or preferably, persulphate - of iron. It becomes black when washed with infusion of galls; blue, by prussiate of potash. [This constitutes colourless ink, which becomes visible when written with on paper containing galls, or tannin, or prussiate of potash.]

7. Mix equal quantities of sulphate of copper and sal ammoniac, and dissolve in water. It becomes yellow when heated.

8. A weak solution of nitrate of mercury. Becomes black by heat.

9. Bice water, or any solution of starch. It becomes blue when washed over with an alcoholic solution of iodine.

10. Lemon juice, milk, juice of onions, and some other liquids, become black when the writing is held to the fire.