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 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.