PHYSIOLOGICAL LIGHT. 423 



of years, is reducible to a simple chemical or physical phenomenon 

 properly so called, or whether there is reason to conclude that it 

 depends upon vital phenomena which should be considered together 

 in a special chapter of general mechanics — physiological mechanics or 

 biological dynamics. 



Part Second. — Special Mechanism of the Photogenic 



Function. 



Since we employ the expression animal heat to designate the heat 

 produced by auimals, so we should understand by physiological light the 

 light generated by the biological activity of living beings. It is dis- 

 tinguished from all other light by its composition, its origin, and the 

 special mechanism by which it is produced. 



I. 



The color of the light may vary according to the species that pro- 

 duces it. In the Photouactcriacew it is sometimes silvery white, some- 

 times bluish or green, sometimes tinged with orange. In the same 

 species it may also change according to the environment. In the solid 

 bouillon of gelatin x>eptone the Photobacterium saroophilum emits a 

 greenish luster, which (manges to a light blue in liquid bouillon. 



In mushrooms, we observe similar variations; the luminosity of 

 Agaricus igneus is bluish, that of Agaricus Gardner! dull green, that of 

 Agaricus olearius and noctilucens white. 



The same remark applies to animals, and, among these, the same indi- 

 vidual is quite often seen to change its color from one moment to another. 

 There have been taken from more than a hundred fathoms depth in the 

 Strait of Skye Pennatulidce or sea-pens that shine with a pale lilac 

 light; in other cases, as in Ophiura and BalanogJossus, the light is a 

 fine emerald green. It is bluish in Lampyra, white with golden glints 

 in Luciola. Two lights of different colors may coexist in the same 

 individual ; certain exotic larvae have a red light near the head and 

 bluish lights along the body. The light may also vary with the meta- 

 morphoses of the individual. The egg and the young larva of Pyro- 

 phorus noctilucus give out pale blue rays, while those emitted by the 

 adult are light green. 



But what is more singular, is to see in the same animal all the colors 

 of the spectrum succeed each other rapidly and without interruption. 

 From all points of the stems and branches of certain Grorgonidre, light 

 may be seen to gush forth in jets and sprays of fire, whose luster dies 

 away, then revives, passing from violet to purple, from red to orange, 

 from bluish to different tints of green, sometimes even to the white of 

 superheated iron. In the same way the Pyrosomata offer a very curi- 

 ous spectacle when heated or strongly excited. Pyrosoma atlanticum 

 becomes at first red, then pink, then orange, afterwards greenish, 



