ANNOTATIONS AND ADDITIONS. 265 



dead Dagysse and Medusae, the sea may perhaps be looked on as a 

 gelatinous fluid, which as such is luminous, distasteful to, and un- 

 drinkable by man, and capable of affording nourishment to many 

 fish. If 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 con- 

 tact with the oxygen of the atmosphere, or of the air held in solu- 

 tion 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 phos- 

 phorescence 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 impreg- 

 nated with gelatinous animal substances ? Perhaps it is only in 

 certain states of the atmosphere that the light-evolving animalcule 

 come in large numbers to the surface of the sea. It has been asked 

 why the fresh water of our marshes, which is filled with polypi, is 

 never seen to become luminous. Both in animals and plants, a 

 particular mixture of organic particles appears to be required in 

 order to favor the production of light. Willow-wood is oftener 

 found to be luminous than oak-wood. In England, experiments have 

 succeeded in making salt water shine by pouring into it the liquor 

 from pickled herrings. It is easy to show by galvanic experiments 

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