Picric acid is formed by the action of nitric acid upon phenol (carbolic acid). Picric acid is a pale yellow crystalline substance sometimes used in dyeing, as it yields a fine pale yellow upon silk. It is principally used in the preparation of some of the "'high" explosives. It does not explode by applying a light or by friction, but when a strong detonating cap is exploded, in a cartridge of picric acid, the latter is caused to explode with terrific violence. The combinations of picric acid with soda and potash are amongst the most powerful ex-ives, but as they sometimes explode spontaneously, they are rarely used.