Note 2 — Chapter VII

There are many stories, from other sources, confirmatory of these noises in mountainous districts. One solution, suggested by Humboldt,—who does not, however, record the fact as of his own observation,—is, that "this curious phenomenon announces a disengagement of hydrogen, produced by a bed of coal in a state of combustion." This solution is applicable only to mountains which contain coal, unless chemical changes in other minerals might be supposed capable of producing a similar effect.