124 EMISSION OF LIGHT 
Medusee upon a tin plate, he observed that when- 
ever he struck this plate with another metal, the 
shghtest vibrations of the tin were sufficient to 
render the animal completely luminous many 
consecutive times. It has been observed also that 
these Medusze emit a more intense phosphores- 
cence when they are placed in a galvanic circuit ; 
but the electric current must not be too powerful. 
The experiments undertaken by M. Suriray a 
Havre, by Professor Hhrenberg at Heligoland, by 
M. de Quatrefages at Boulogne, and by Dr. Ver- 
haghe at Ostend, have added considerably to our 
knowledge of the emission of hg¢ht by Noctiluca. 
All mechanical or chemical agents that bring 
about a contraction of tissue in these animalcules 
excite their phosphoric quality. A drop of weak 
acid or alcohol, a shock given to the glass 
which contains them, immediately renders every 
individual luminous. If a few teaspoonfuls of 
Noctiluca be collected upon a filter, the heht they 
emit is powerful enough to enable us to read at a 
distance of nine inches and a half. When the 
bulb of a small and very sensitive thermometer is 
plunged into this little heap of Noctiluca, it is 
found, that although these small beings are in full 
life, not the shghtest elevation of temperature can 
be observed during the emission of their light. 
A curious observation has been made by Pro- 
fessor Ehrenberg. By submitting his “ Mam- 
