SECT. XXIX. VOLTAIC CONDUCTION. 309 



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. 



Static electricity, on account of its high tension, passes through 

 water and other liquids as soon as it is formed, whatever the 

 length of its course may be. Voltaic electricity, on the contrary, 

 is weakened by the distance it has to traverse. Pure water is a 

 very bad conductor ; but ice absolutely stops a current of Voltaic 

 electricity altogether, whatever be the power of the battery, 

 although static or common electricity has sufficient power to 

 overcome its resistance. Dr. Faraday has discovered that this 

 property is not peculiar to ice ; that, with a few exceptions, 

 bodies which do not conduct electricity when solid acquire that 

 property, and are immediately decomposed, when they become 

 fluid, and, in general, that decomposition takes place as soon as 

 the solution acquires the capacity of conduction, which has led 

 him to suspect that the power of conduction may be only a con- 

 sequence of decomposition. 



Heat increases the conducting power of some substances for 

 Voltaic electricity, and of the gases for both kinds. Dr. Faraday 

 has given a new proof of the connexion between heat and elec- 

 tricity, by showing that, in general, when a solid, which is not a 

 metal, becomes fluid, it almost entirely loses its power of con- 



