338 FOKMATION OF CRYSTALS. [SECT. xxix. 



respective poles. According to Dr. Faraday, electro-chemical 

 decomposition is simply a case of the preponderance of one 

 set of chemical affinities more powerful in their nature over 

 another set which are less powerful. The great efficacy of 

 Voltaic electricity in chemical decomposition arises from 

 the continuance of its action ? and its agency appears to be 

 most exerted on fluids and substances which, by conveying 

 the electricity partially and imperfectly, impede its 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, pre- 

 cisely 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 already mentioned 

 as the great agent in the formation of solids. Both light and 

 motion favour crystallization. Crystals which form in dif- 

 ferent 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. Light and motion are intimately connected with 

 electricity, which may therefore have some influence on the 

 laws of aggregation ; this is the more likely, as 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 1 



The deposition of metal from a metallic solution by galvanic 



