- [Footnote: one rubs a board with part of a Medusa hysocella, the part
so rubbed regains its luminosity on friction with a dry finger. On my passage to South America I sometimes placed a Medusa on a tin plate. When I struck another metallic substance against the plate, the slightest vibrations of the tin were sufficient to cause the light. What is the manner in which in this case the blow and the vibrations act? Is the temperature momentarily augmented? Are new surfaces exposed? or does the blow press out a fluid, such as phosphuretted hydrogen, which may burn on coming into contact with the oxygen of the atmosphere or of the air held in solution by the sea-water. This light-exciting influence of a shock or blow is particularly remarkable in a "cross sea," i. e. when waves coming from opposite directions meet and clash.
I have seen the sea within the tropics appear luminous in the most different states of weather; but the light was most brilliant when a storm was near, or with a sultry atmosphere and a vaporous thickly-clouded sky. Heat and cold appear to have little influence on the phenomenon, for on the Banks of Newfoundland the phosphorescence is often very bright during the coldest winter weather. Sometimes under apparently similar external circumstances the sea will be highly luminous one night and not at all so the following night. Does the atmosphere influence the disengagement of light, or do all these differences depend on the accident of the observer sailing through a part of the sea more or less abundantly impregnated with gelatinous animal substances? Perhaps it is only in certain states of the]*
