Dissolve oxide of cobalt in acetic acid, to which add a little nitre;
write with this solution, hold the writing to the fire, and it will be
of a pale rose colour, which will disappear on cooling.
Dissolve equal parts of sulphate of copper and muriate of ammonia in
water; write with the solution, and it will give a yellow colour when
heated, which will disappear when cold.
Dissolve nitrate of bismuth in water; write with the solution, and
the characters will be invisible when dry, but will become legible on
immersion in water.
Dissolve, in water, muriate of cobalt, which is of a bluish-green
colour, and the solution will be pink; write with it, and the
characters will be scarcely visible; but, if gently heated, they will
appear in brilliant green, which will disappear as the paper cools.