One cause of the great Heat of many of our deep Mines, which appears to
have been entirely lost sight of, is the chemical action going on upon
large masses of pyritic matter in their vicinity. The heat, which is so
oppressive in the United Mines in Cornwall that the miners work nearly
naked, and bathe in water at 80° to cool themselves, is without doubt
due to the decomposition of immense quantities of the sulphurets of
iron and copper known to be in this condition at a short distance from
these mineral works.--_R. Hunt, F.R.S._