28 L. H. Borgström och J. Dannholm (LVIII 



The specific resistance was calculated from the differences. 



— 2o. The resistance was determined not between the 

 electrodes, which carried the main circuit to the ends of 

 the mineral-rod, but between secondary electrodes, parasitic 

 electrodes, and measured according to the method of the 

 double bridge of T h o m s o n (Lord K e 1 v i n). — 3o. With 

 the aid of parasitic electrodes a secondary circuit was formed 

 between two points on the rod some distance from the main 

 electrodes at the ends of the rod. The strength of this current 

 and that of the main current were measured and the resistance 

 between the two points on the rod calculated. — All three 

 methods gave good results as stated in Table VII page 22. 



— The resistance in the contacts between the ends of the 

 mineral-rod and the amalgamated copper electrodes of the 

 main circuit were estimated (Table IV page 15) and found 

 to be nearly equal to the resistance of the shorter (2 — 3 mm) 

 rods; it is thus by no means to be overlooked. — The speci- 

 fic resistance of the natural arsenides whitneyite Cu^As, and 

 algodonite, Cuq As are much smaller than the resistance of the 

 molten and solidified materials. The difference is very great 

 and therefore it can safely be assumed that the molten 

 material has a different mineralogical composition. — The 

 exact determination of the electric conductivity of mine- 

 rals, especially the ore minerals, may be of more use for 

 mineralogy and geology than has hitherto been supposed. 



