102 EMISSION OF LIGHT BY 



When these dead fish are placed in sea-water, 

 they render it luminous in the course of a few 

 days, the phosphorescence of the sea is however 

 owing to a different cause, and the water then 

 shines in a uniform, manner, i.e. everywhere with 

 equal intensity : if it be passed through a filter it 

 continues to shine as before. These facts prove 

 clearly that this singular phosphorescence is not 

 owing to any luminous animalcules. 



Water that has been rendered luminous by 

 dead fish loses its transparency, becomes milky, 

 and acquires a repulsive odour; in the space of 

 four or five days it ceases to be luminous. 



Hulme, who has made numerous observations 

 on this particular case of phosphorescence, says 

 that the luminous greasy substance of the herring 

 soon loses its phosphoric properties in pure water. 

 Alcohol, acids, and alkalis also prevent its shining. 

 Common salt and honey appear, on the contrary, 

 to assist this phosphorescence. Sometimes also, 

 when the latter has been extinguished by one 

 means or another, it can be brought back again : 

 thus, in one of Hulme' s experiments, twenty -four 

 grammes of sulphate of magnesia, dissolved in 

 twenty-one grammes of water, and mixed with 

 the luminous substance of the mackerel, com- 

 pletely extinguished its light ; but if to this mix- 

 ture six times its volume of water were added, it 

 became luminous again. The same observer also 



