(il(j SUPPLEMENTARY NOTES. 
why the sulphuret of copper commonly occurs in metallic veins 
nearer the surface than the yellow bi-sulphuret, where it is ex- 
posed to the action of water and of ferruginous matter, as indi- 
cated by the *' gossan,'' or oxide of Iron, which occurs in the 
upper regions of Copper mines in Cornwall. Mr. R. W. Fox re- 
ferred also to his experiments on the electro-magnetic condition 
of metallic veins, and adduced proofs of the electricity which he 
had detected in them, being independent of accidental influence ; 
indeed, he obtained very decided voltaic action when a piece of 
sulphuret, and another of yellow bi-sulphuret of copper were 
dipped in water, taken from a mine, the former being electro- 
positive with respect to the latter. This experiment shows that 
the voltaic action between different metallic lodes, and different 
parts of the same lode, must be very great. He was induced to 
commence his electro-magnetic experiments in mines in conse- 
quence of the analogy which he thought he perceived in mineral- 
veins to voltaic combinations. 
In another experiment Mr. R. W. Fox has substituted the 
sulphuret or vitreous copper ore for the piece of Zinc in one of 
the cells, all other circumstances being the same as before de- 
scribed, and in a few weeks the yellow bi-sulphuret of copper in 
the other cell was covered with a thin coating of the sulphuret of 
that metal. He has also found that sulphuretted hydrogen is 
copiously evolved when yellow copper ore is placed in a solution 
of sulphate of Zinc or of Iron in one of the cells, and connected, 
by means of a wire, with a piece of Zinc in the water of the other 
cell. As sulphuretted hydrogen has the property of precipi- 
tating most of the metals from their solutions, in the form of sul- 
phurets, this experiment seems to point at an agent which may 
have produced many of the metallic sulphurets. See vol. ii. 
P. 108. Note. 
In a subsequent communication to the Geol. Soc. of London, 
January, 1837, Mr. Fox observes, " I imagine that I see more and 
more reason to believe, that the Eastward and Westward ten- 
dency of metallic veins, must be ascribed to the electro-magnetic 
influence of the earth. In some parts of the world there may be 
considerable deviations from this bearing, which may be owing 
to local circumstances; but the coincidence in their direction, 
generally speaking, is so great and decided as clearly to indicate 
