I have seen the ocean, in the tropics, luminous in the most opposite kinds of weather, but most strongly so before a storm, or in a sultry and hazy atmosphere with thick clouds. Heat and cold appear to exercise but little influence on this phenomenon, for, on the Bank of Newfoundland, the phosphorescence is frequently very brilliant in the severest winter. Occasionally, too, the sea will be highly luminous one night, and not at all so on the following, notwithstanding an apparent identity of external conditions. Does the atmosphere favour this development of light? or do all the differences observed during this phenomenon depend on the accidental circumstance of the sea being more or less impregnated, in some parts, with the gelatinous portions of mollusca? Perhaps these phosphorescent social animalcules only rise to the surface under certain conditions of the atmosphere. It has been asked, why our fresh-water swamps which are filled with polyps are not phosphorescent. It would appear that, both in animals and plants, a peculiar mixture of organic particles favours this development of light; thus, for instance, the wood of the willow is more frequently found to be luminous than that of the oak. In England, salt-water has been rendered luminous by mixing herring-brine with it; indeed, it will be easy for any one to convince himself by galvanic experiments, that the luminosity of living animals depends on nervous irritation. I have observed strong phosphorescence emitted from a dying _Elater noctilucus_, on touching the ganglion of its fore leg with zinc and silver. Medusæ also occasionally emit a stronger light at the moment the galvanic circuit is completed.[KM] Footnote 76: p. 213—“_Which inhabits the lungs of the Rattlesnake of the tropics_.”