Electro-magnetic properties in the mines of Cornwall. 139 



between the electricity: and temperature at the same level, but then 

 the differences of temperature are comparatively small. Nor does 

 the electricity appear to be influenced by the presence of the work- 

 men and candles, or by the explosion of gunpowder, although some 

 veins of copper ore were blasted on different occasions in the imme- 

 diate vicinity of the copper plates. At a very productive copper 

 vein in Great St. George mine, the ground is so soft that gunpow- 

 der is not used : yet the needle was powerfully acted upon by the 

 electricity it contained. On this occasion as well as on some others, 

 the galvanometer remained at the surface, the wires being passed 

 down through the shafts ; and in this manner it was sometimes found 

 that the electricity acted with considerable energy, so as even to 

 cause the needle to revolve with some velocity. 



In connection with the electricity of veins, the author deemed it 

 desirable to ascertain the relative power of conducting galvanic elec- 

 tricity possessed by many of the metalliferous minerals; and it ap- 

 peared to be in about the following order, viz. 



Conductors. 



Copper nickel, purple copper, yellow sulphuret of copper, vit- 

 reous copper, sulphuret of iron, arsenical pyrites, sulphuret of lead, 

 arsenical cobalt, crystallized black oxide of manganese, tennantite, 

 fahlerz. 



T^ery imperfect conductors. 



Sulphuret of molybdenum, sulphuret of tin, or rather bell-metal ore, 



J^on-conductors. 



Sulphuret of silver, sulphuret of mercury, sulphuret of antimony, 

 sulphuret of bismuth, cupriferous bismuth, realgar, sulphuret of 

 manganese, sulphuret of zinc. 



Minercd combinations of metals ivith oxygen and with acids. 



Amongst the rocks prevalent in Cornwall, clay slate or " killas" 

 seemed to possess the property of conducting common electricity, in 

 a shght degree, but only in the direction of its cleavage, perhaps 

 owing to the moisture it retained. 



These facts are mentioned in some detail, because it is curious to 

 observe that the conducting power of metallic ores appears to have 

 no reference to any of the electrical or other properties of the metals 



