170 PHOSPHORESCENCE OF SEA. 



and may easily enough be witnessed ; as if a 

 vessel is filled with water drawn from the surface 

 of the sea on a calm day, there are generally a few 

 Medusae in it. But if there should be none, a 

 little work with a gauze net will secure plenty. 



Very many of these Acalephs are phospho- 

 rescent, and through their instrumentality the 

 sea appears at night as if filled with fire. This 

 property is especially shown when a little breeze 

 ripples the surface, or a boat dashes the water 

 aside. In the latter case the oars appear to throw 

 from tnem torrents of fire, and a blazing line 

 marks the direction which the boat has taken. 

 The chief cause of this phosphorescence is a very 

 tiny creature, called Noctiluca miliaris. If a vase 

 be filled with sea-water and placed in the dark, it 

 will emit small sparkles of light whenever it is 

 tapped, or even when the foot is stamped on the 

 ground. This phenomenon is more exemplified 

 at the sides of the jar, and but a very few sparks 

 are perceptible at the bottom. Each spark is 

 caused by a Noctiluca ; and if it is wanted for 

 examination, it may be caught in a glass tube, 

 in the manner employed for microscopic animal- 

 cules, and brought under the necessary magnify- 

 ing power. Its natural size is about half that of 

 a common mustard-seed. When submitted to 



