CRYSTALLIZATION PRODUCED BY VOLTAIC ACTION. 



417 



This remarkable fact can only be observed with an electrical apparatus of 

 very feeble tension, inasmuch as, when this tension possesses a certain 

 energy, the dissolved metal goes always to the negative pole, be the wire 

 or plate of metal which is plunged into the solution what it may. 



To what cause are we, in this instance, to ascribe the predilection 

 of a metal, combined with an acid, for a plate of the same metal ? The 

 force of cohesion, Avhatever it may be, is the only influence to which 

 it can be attributed ; for that principle must be supposed to act with 

 greater force on similar than on dissimilar molecules. In this case, tlie 

 force of cohesion added to that of the electric current should determine 

 the precipitation. It must not be forgotten however that the chemical 

 action of the solution on the positive wire is also powerfully conducive 

 to the production of the general effect. 



If it be desired to obtain continuous effects with the thermo-electric 

 apparatus, the copper ring into which the platina ring is passed must 

 be renewed from time to time, because, at a certain stage of the experi- 

 ments, the copi^er being entirely oxidized, the continuity is broken and 

 the electro-chemical efl'ects consequently cease. 



An apparatus with a platina and an iron wire has not sufficient action 

 to produce decompositions. This negative effect is undoubtedly to be 

 attributed to the singular electric properties of iron. 



The apparatus we are about to describe is intended to produce slow 

 and continuous electro-chemical actions. 



Take two small glass jars (fig. 1); having poui'ed into the one some 



Fig.i. 



nitric acid, and into the other some potash dis-olved in water, we esta- 

 blish a communication between them by means of a bent glass tube 

 filled with potters' clay moistened with a solution of nitrate of potash or 

 chloride of sodium, and then plunge into each liquid a plate of plafma 

 fixed to the extromitv of a wire of the same metal. At the free end of 



