130 Professor Bischof o?i the Origin of 



stance would be deposited from the same watery solution, 

 according as it came in contact with this or that rock. 

 Were they, for example, felspar and mica, which induced, 

 by exchange, the deposit of silica or any other substance, 

 such a deposit could only take place when the solution flowed 

 through fissures of a crystalline rock, containing these mi- 

 nerals. Should the veins run into other rocks, in which 

 these minerals were wanting, there could no deposit of such 

 substances take place. 



We must not overlook the essential circumstance, that the 

 weakest affinities would come into play in processes which, 

 like those by which the vein-stuffs were introduced into the fis- 

 sures, went on with such extreme slowness. Several instances, 

 in the circle of chemical phenomena, indicate this. Thus I 

 found in a wooden tub, bound with iron hoops, in a manufac- 

 tory of sulphate of copper (green copperas), deposits of solid 

 metallic copper, of which some had a weight of several 

 pounds. They fixed themselves on two staves, between 

 which a plate of copper had forced itself. It was manifestly 

 the iron hoop which had, in this case, effected the reduction 

 of the copper. It was not, however, a simple precipitation 

 of the copper by iron, but, as in the so-called metallic vegeta- 

 tions, a galvanic or electric effect ; as, namely, a thin metallic 

 plate of copper had been deposited in the interval between 

 the two staves by simple chemical action, a metallic contact 

 was established between the copper and the iron of the 

 hoop, and now the precipitation of copper on copper, which 

 formed the negative pole, went slowly forwards, without the 

 iron further influencing the process directly. Had the effect 

 resulted entirely from the iron hoop, it must have been pei*- 

 fectly eaten through in this part, Avhich was not the case. 

 If we consider that, in this process, through the interstice of 

 two staves, which was so narrow that no fluid could run out, 

 a chemical action nevertheless took place, which, in the lapse 

 of some years, effected the deposit of considerable masses of 

 metallic copper, it cannot surprise us to observe similar ap- 

 pearances in metalliferous veins. We can compare the effec t 

 of the iron hoop with that of substances which had been con- 

 tained in the adjacent rock, and which communicated with 



