370 INTRODUCTION TO CONCHOLOGY. 



The luminous water, when poured upon fresh calcined 

 gypsum, rock crystal, or sugar, becomes more vivid. Milk 

 rendered luminous by the Fholas, loses its shining quality 

 when mixed with sulphuric acid ; but regains it on the addi- 

 tion of oil of tartar. Coloured substances are differently and 

 powerfidly affected by it : white appears to imbibe and emit 

 the greatest quantity ; yellow and green in less proportions ; 

 red will emit hardly any light ; violet least of all. 



A single P/iolas will render seven ounces of milk so 

 beautifully luminous, that surrounding objects are clearly 

 visible by its light. This luminous quality entirely disappears 

 when the milk which contains it is excluded from the air ; 

 but again revives on exposure to the atmosphere. In the 

 exhausted receiver of an air-pump, the Pholas loses his light. 



The reason for this remarkable provision, as well as the 

 purpose to which it is applied in the animal economy, are 

 unknown. It is one of those extraordinary facts in natural 

 history, which, like the playful meteors that often beautifully 

 enliven the solitary woods with their agile and wandering 

 lights, repeatedly invite the traveller to an unsatisfactory 

 pursuit, and as frequently elude his vigilance. 



The Greek word ][^lioleo, from which the generic appella- 

 tion of this extraordinary shell-fish is derived, signifies, ^^ I 



