secret message. He laid several sheets of note paper on each other and
wrote on the uppermost with a pencil; then selected one of the under
sheets, on which no marks of the writing were visible. On exposing
this sheet to the vapor of iodine for a few minutes it turned
yellowish and the writing appeared of a violet brown color. On further
moistening the paper it turned blue, and the letters showed in violet
lines. The explanation is that note paper contains starch, which under
pressure becomes "hydramide," and turns blue in the iodine fumes. It
is best to write on a hard surface, say a pane of glass. Sulphuric
acid gas will make the writing disappear again, and it can be revived
a second time.

One of the simplest secret writings, however, to which Professor Gross
of Germany calls attention is the following:

Take a sheet of common writing paper, moisten it well with clear
water, and lay it on a hard, smooth surface, such as glass, tin,
stone, etc. After removing carefully all air bubbles from the sheet,
place upon it another dry sheet of equal size and write upon it your
communication with a sharp-pointed pencil or a simple piece of pointed
hardwood. Then destroy the dry paper upon which the writing has been
done, and allow the wet paper to dry by exposing it to the air (but
not to the heat of fire or the flame of a lamp). When dry, not a trace
of the writing will be visible. But on moistening the sheet again with
clear water and holding it against the light, the writing can be read
in a clear transparency. It disappears again after drying in the air,
and may be reproduced by moistening a great number of times. Should
the sheets be too much heated, however, the writing will disappear,
never to reappear again. This system is used extensively in Germany.