progress. But it is now proved to be as efficacious in the composition as in the decomposition or analysis of bodies. It had been observed that, when metallic solutions are subjected to galvanic action, a deposition of metal, sometimes in the form of minute crystals, takes place on the negative wire. By extending this principle, and employing a very feeble Voltaic action, M. Becquerel has succeeded in forming crystals of a great proportion of the mineral substances, precisely similar to those produced by nature. The electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles, when in solution, within the narrow sphere of molecular attraction. Both light and motion favour crystallization. Crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light; and it is well known that still water, cooled below 32°, starts into crystals of ice the instant it is agitated. A feeble action is alone necessary, provided it be continued for a sufficient time. Crystals formed rapidly are generally imperfect and soft, and M. Becquerel found that even years of constant Voltaic action were necessary for the crystallization of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond! The deposition of metal from a metallic solution by galvanic electricity has been most successfully applied to the arts of plating and gilding, as well as to the more delicate process of copying medals and copper plates. Indeed, not medals only, but any object of art or nature, may be coated with precipitated metal, provided it be first covered with the thinnest film of plumbago, which renders a non-conductor sufficiently conducting to receive the metal. Photo-galvanic engraving depends upon this. Gelatine mixed with bichromate of potash, nitrate of silver, and iodide of potassium, is spread over a plate of glass, and when dry a positive print is laid upon it with its face downwards, which, when exposed to the sun, leaves its impression. When soaked in water the gelatine swells around all those parts where the light had fallen, thus forming an intaglio, a cast of which is taken in gutta-percha, which is then coated with copper by the electro process, whence a copper plate in relief is obtained.