360 Experimental Researches in Electricity. 



have to certain points of geological science, those, namely, 

 connected with the occurrence of crystallised minerals in 

 metamorphic and igneous rocks. That the occurrence of 

 simple minerals in rocks that have been subjected to the 

 action of heat, is intimately related to, or perhaps depend- 

 ent upon, the action of polar electrical forces, we entertain 

 the firmest conviction, although we still require experimental 

 evidence to complete the chain of induction in favour of this 

 idea, which receives some confirmation from the appearance 

 of several natural mineral bodies in the list Faraday gives 

 of those he has found to be subject to the new law of con- 

 duction he has establishedo It would occupy, however, too 

 much of our space and time to enter in any detail on this 

 point at present, and as without such detail it would be vain 

 to attempt to do justice to it, we prefer passing it by with- 

 out farther remark. 



The fourth series is concluded by some general remarks 

 on conduction ; and it will suffice for us now to give the sum- 

 mary of the conditions of electric conduction in bodies, with 

 which Faraday terminates his paper. 



" All bodies conduct electricity in the same manner, from 

 metals to lac and gases, but in very different degrees. 



" Conducting power is in some bodies powerfully increased 

 by heat, and in others diminished ; yet without our perceiv- 

 ing any accompanying essential electrical difference either 

 in the bodies, or in the changes occasioned by the electri- 

 city conducted. 



" A numerous class of bodies, insulating electricity of low 

 tension, when solid, conduct it very freely when fluid, and 

 are then decomposed by it. 



" But there are many fluid bodies which do not sensibly 

 conduct electricity of this low intensity ; there are some 

 which conduct it, and are not decomposed ; nor is fluidity 

 essential to decomposition. 



"There is but one body yet discovered (per-iodide of mer- 



