Many years ago, Mr. R. W. Fox, from theory entertaining a belief
that a connection existed between voltaic action in the interior of
the earth and the arrangement of metalliferous veins, and also the
progressive increase of temperature in the strata as we descend from
the surface, endeavoured to verify the same from experiment in the mine
of Huel Jewel, in Cornwall. His apparatus consisted of small plates
of sheet-copper, which were fixed in contact with a plate in the veins
with copper nails, or else wedged closely against them with wooden
props stretched across the galleries. Between two of these plates,
at different stations, a communication was made by means of a copper
wire 1/20th of an inch in diameter, which included a galvanometer
in its circuit. In some instances 300 fathoms of copper wire were
employed. It was then found that the intensity of the voltaic current
was generally greater in proportion to the greater abundance of copper
ore in the veins, and in some degree to the depth of the stations.
Hence Mr. Fox’s discovery promised to be of practical utility to the
miner in discovering the relative quantity of ore in the veins, and the
directions in which it most abounds.
The result of extended experiments, mostly made by Mr. Robert Hunt,
has not, however, confirmed Mr. Fox’s views. It has been found that
the voltaic currents detected in the lodes are due to the chemical
decomposition going on there; and the more completely this process
of decomposition is established, the more powerful are the voltaic
currents. Meanwhile these have nothing whatever to do with the increase
of temperature with depth. Recent observations, made in the deep mines
of Cornwall under the direction of Mr. Fox, do not appear consistent
with the law of thermic increase as formerly established, the shallow
mines giving a higher ratio of increase than the deeper ones.