There are some articles employed for this purpose, which are rendered visible by the addition of a substance which acts chemically. The materials of writing ink may, for example, be employed in a separate state. Invisible words may be first written with a solution of the sulphate of iron. If a rag, dipped in a decoction of galls, be drawn ever them, they become immediately legible. If this be afterwards rubbed over with sulphuric acid, it is effaced.
But the application of a saturated solution of potassa will make it re-appear like yellow writing.
The golden sympathetic ink consists of a solution of gold in nitro•muriatic acid, diluted with six times its quantity of water. Letters traced with this are invisible ; but when a similar solution of tin is applied to them, the writing ap pears in the form of beautiful purple letters. Nitromuria tic acid is now capable of effacing them, and the re-appli cation of the muriate of tin will restore them. Letters made with the muriate of gold, indeed, become spontane ously visible when exposed to the air. This, however, re quires several days, and, if kept closely shut up, they re • main invisible for two or three months. The acid evapo rates, and leaves a violet oxide or submuriate. Nitrate of silver affords invisible letters, which become black by long exposure.
There are many sympathetic inks which are renderer' visible by exposure to a fire. Solutions of muriate of am monia, and various other neutral salts, act on paper by means of heat, in such a way as adapts them to this use ; but the letters become, in process of time, confused and illegible.
The best sympathetic inks are those made from ores of arsenic, bismuth, or cobalt. Diluted nitric acid is poured on arsenic ore, and afterwards carefully decanted, treated with nearly half the quantity of dried muriate of soda, and evaporated. Letters or figures formed with this are invi sible till held near a fire, which renders them visible, and of a beautiful bluish green colour. This disappears again when it is removed from the fire. Alum, with the sulphate of soda, used instead of muriate of soda, renders the substance red. Borate of soda, or nitrate of potassa, also makes the letters appear red.
The nitro-muriate of cobalt forms a similar ink, which appears on exposure to heat, and disappears in the cold. The heat applied to it, however, must not exceed a cer tain strength, otherwise the letters become permanently visible both in heat and cold. These inks are employed for making amusing landscapes, in which the trees ac quire a summer foliage as often as they are brought near a fire.
A sympathetic ink may be obtained from fresh urine, evaporated, then dissolved in nitric acid, and saturated with ammonia or its carbonate. Lines drawn with this be come visible, of a fine red, on exposure to a gentle heat. Sulphate of zinc applied to them gives them a rich yellow. This ink must be applied, however, immediately after it is formed. It becomes deteriorated in so short a time as twenty minutes. (IL D.)