114 EMISSION OF LIGHT 
mous layer of shining phosphorus or molten lava. 
These singular productions of nature are met with 
between 19° and 20° of longitude east of Paris, 
and 3° and 4° north latitude. 
The no less curious animals belonging to the 
genus Salpa (fig. 18, Salpa cristatu: 1, an 1s0- 
lated individual; 2, five Salpe united as they 
swim), also classed in the Tunicata, abound in the 
Fig. 13. 
Mediterranean and warmer parts of the ocean. 
They are often phosphorescent. They also swim, 
adhering together in vast numbers; their phos- 
phorescence resembles the hght of the moon on 
the still waters, and they give rise to what is termed 
by the French a mer de lait, or sea of milk. 
Sir Joseph Banks, in his voyage from Madeira 
to Rio Janeiro, discovered the little crab Cancer 
fulgens, a species said to be very phosphorescent. 
In nearly the same latitude that this discovery 
was made, Medusa pellucens was met with; its 
phosphorescence is described as resembling a 
flash of hghtnine. 
