Ovid, in his "Art of Love," teaches young women to deceive their
guardians by writing their love letters with new milk, and to make the
writing appear by rubbing coal dust over the paper. Any thick and
viscous fluid, such as the glutinous and colorless juices of plants,
aided by any colored powder, will answer the purpose equally well. A
quill pen should be used.

The most common method is to pen an epistle in ordinary ink,
interlined with the invisible words, which doubtless has given rise to
the expression, "reading between the lines," in order to discover the
true meaning of a communication. Letters written with a solution of
gold, silver, copper, tin, or mercury dissolved in aqua fortis, or
simpler still of iron or lead in vinegar, with water added until the
liquor does not stain white paper, will remain invisible for two or
three months if kept in the dark; but on exposure for some hours to
the open air will gradually acquire color, or will do so instantly on
being held before the fire. Each of these solutions gives its own
peculiar color to the writing--gold, a deep violet; silver, slate; and
lead and copper, brown.

There is a vast number of other solutions that become visible on
exposure to heat, or when having a heated iron passed over them; the
explanation is that the matter is readily burned to a sort of
charcoal. Simplest among these are lemon juice or milk; but the one
that produces the best result is made by dissolving a scruple of
salammoniac in two ounces of water.

Several years ago Professor Braylant of the University of Louvain
discovered a method in which no ink at all was required to convey a