356 GEOLOGY OF THE COMSTOCK LODE. 



available it would be of little use, however, to attempt to assign reasons for 

 the remaining variations. A matter of greater importance is that the gen- 

 eral character of the curves, as derived from the two series of results, is 

 essentially the same.^ 



UNAVOIDABLE ERRORS AND MISCELLANEOUS CRITICISMS. 



Moisture in the rocks. — Bv far thc most seHous difficulty encountered in en- 

 deavoring to interpret the results obtained, is that due to the difference of 

 potential of two liquids in contact. The conductivity of rocks is, as has 

 been seen, largely, if not wlioUy, to be ascribed to the presence of moisture 

 in their pores. This moisture unquestionably holds saline matter in solu- 

 tion. Moreover, it is altogether probable that the solution in one rock of a 

 particular structure is in general different from that in another of different 

 structure and many hundred feet distant from the former, even if the com- 

 position of both is essentially the same. In tapping two points at some dis- 

 tance apart bv the aid of two metals (plates or gads) supposed identical in 

 every respect, two members of the continuous sequence of solutions con- 

 tained in the rocks are, in fact, put in metallic contact. The difference of 

 potential thvis obtained would be that due to the resultant action of the 

 series of liquids included between the points. This electromotive force is, 

 however, principally dependent on the extreme members of the series, i. e., 

 those at the points tapped ; and in the present investigation it was hoped 

 that the discrepancy thus arising might be very largely eliminated by put- 

 ting the same liquid in both holes, and by exchanging not only the metallic 

 terminals — amalgamated zinc — butalso theterminal solutions (zinc sulphate). 

 Hence the "bag" form of the terminal. 



It was thought not superfluous to test the matter with the aid of the 

 contact bags themselves; all the more as it would thus appear to what 

 extent the results obtained with the latter are trustworthy. The two liquids, 

 whose electromotive force was to be measured, were separated from one 

 another by a porous septum of animal membrane. As in the mines, the 

 terminal bags were exchanged. In passing them out of the first liquid into 

 the second, care was taken to wipe off the liquid adhering to the outside. 



' Compare Figs. 30 and 32. 



