Cobalt

a metal resembling silver in appearance; but with a slight trace of red. It is brittle and highly magnetic. It is harder than iron, but melts more easily and is a trifle heavier. Cobalt occurs in nature almost always in company with nickel, and is produced with it. Compounds of cobalt, called cobalt salts, have a peculiar way of changing color. When dry they are blue, and when wet they are red. Ink made of a cobalt salt is accordingly red, and produces red writing; but if the writing be dried before a fire until it loses its moisture, the writing turns blue. Exposure to vapor brings back a red color. If the ink be made very weak the red will not show, but heat will develop the blue. In this way secret correspondence has not infrequently been carried on. A letter in ordinary ink is sent with a second letter in invisible ink written between lines. Servants and spies not in the secret were satisfied with reading the letter written in black ink; while that written in sympathetic was unsuspected, save by the person for whom it was intended. Cobalt is a fast mineral dye, giving its name to cobalt blue. It is used in the manufacture of stained glass, paints, colored porcelain, etc. Cobalt blue is unrivaled for beauty and intensity. Large deposits of cobalt have recently been discovered in Canada. The output for 1909 was 48,000 tons, valued at $18,700,000. See Ink