The solutions used should be so nearly colorless that the writing cannot be seen till the agent is applied to render it visible. Sympathetic inks are of three general classes.
Inks that Appear through Heat.
1. Write with a concentrated solution of caustic potash. The writing will appear when the paper is submitted to strong heat.
2. Write with a solution of ammonium hydrochlorate, in the proportion of 15 parts to 100. The writing will appear when the paper is heated by holding it over a stove or by passing a hot smoothing iron over it.
3. A weak solution of copper nitrate gives an invisible writing, which becomes red through heat.
4. A very dilute solution of copper perchloride gives invisible characters that become yellow through heat.
5. A slightly alcoholic solution of copper bromide gives perfectly invisible characters which are made apparent by a gentle heat, and which disappear again through cold.
6. Write upon rose colored paper with a solution of cobalt chloride. The invisible writing will become blue through heat, and will disappear on cooling.
7. Write with a solution of sulphuric acid. The characters will appear in black through heat. This ink has the disadvantage of destroying the paper. (See the caution given on [page 9].)