134 SEVENTH REPORT—1837. 
than in copper veins included in the lower rocks, such as granite 
and “ sillas’’ or clay slate. And here it may be remarked that 
the sulphurets of copper are more electro-negative than galena, 
which circumstance may have had some influence on the results. 
I wished to have made experiments on the electricity of some 
of the coal-beds which have been traversed and charred by the 
great basaltic dyke in the county of Durham, but my time and 
engagements did not well admit of my doing so. It is well 
known, that when coal is reduced to the state of a cinder it be- 
comes a good conductor of voltaic electricity, although coal, in 
its natural state, does not possess this property, or even anthra- 
cite. A friend of mine having kindly sent me some specimens 
of the altered coal taken from Cockfield Fell Colliery, I found 
that most of them were incapable of conducting voltaic electri- 
city, which unexpected circumstance may, perhaps, be attri- 
buted to their having undergone a degree of vitrification,—pene- 
trated, possibly, by some siliceous matter, which their appearance 
indicated; and I am rather confirmed in this opinion from 
having since found that one of the pieces of native cinder from 
the same place is as good a conductor of electricity as coke, and 
it has a less vitrified appearance than the others. Here then 
we have the evidence of electricity in favour of the powerful ac- 
tion of the heated basalt on the contiguous coal deposits. 
I have, on various occasions, endeavoured to show that the 
high temperature observed in the lowest parts of deep mines is 
in a great degree independent of accidental or extraneous causes 
not existing in the earth itself, and, indeed, that it is more often 
diminished by them than the reverse. It occurred to me that 
this point might be decided by burying the bulbs of different 
thermometers at various depths below the deepest excavations 
of mines, and I am indebted to the agents of Levant Tin and © 
Copper mine, and of the Consolidated Copper mines, for having 
carried this plan into effect for me in their respective mines. 
The former mine is situated on the coast, in the parish of St. 
Just, and is worked in granite and “killas.”’ Its depth is about 
230 fathoms from the surface, and 200 fathoms below the level 
of the sea. A thermometer four feet long, and inclosed in a 
brass tube, had its bulb sunk in a hole three feet beneath the 
““ sump,” or bottom of the deepest shaft, whilst another shorter 
thermometer was placed very near it, with its bulb inserted in 
a hole only about an inch deep. The former, which may be 
distinguished as No. 1, indicated a temperature of 80°, and the 
latter (No. 2) of 78°°5, both of them having been previously com- 
pared with a standard thermometer, and the needful correction 
applied. This part of the mine is in granite. The thermome- 
ter was afterwards placed in like manner in “ killas,’’ at the 
