REPTILES AND SNAKE-STONES. 19 



" cobra stone " was a semi-transparent, water-worn 

 pebble of yellowish colour about the size of a large 

 pea, which in the dark, when previously warmed, 

 emitted a greenish, phosphorescent light. I found 

 it to be chlorophane, a rare variety of fluor spar. 

 The mystery is not difficult to explain. Cobras feed 

 on insects, and seem to have an especial liking for 

 fire-flies. I have often for hours watched the snakes 

 in the grass catching the fire-flies, darting about 

 here and there, a process which requires considerable 

 exertion. Only the male fire-flies fly about, and a 

 close observer will notice that a constant swarm of 

 the male insects will fly near the females, which sit 

 on the ground and emit an intermittent glowing 

 light. The cobra uses his phosphorescent stone as 

 a decoy for the fire-flies. No doubt the snake made 

 the discovery by accident, night after night, perhaps, 

 noticing how the fire-flies gathered about the shining 

 pebble. Several snakes gathered, and it would re- 

 quire no great reasoning powers for the cobra to 

 learn that the position of vantage was that nearest 

 the pebble. Competition would lead to the snake's 

 seizing and carrying off the treasure, and habit has 

 become hereditary. 



Another kind of " snake-stones," adder-gems, ovum 

 anguinum or snake eggs, enter into the ancient 

 superstitions of our own country. Borlase tells us * 



* " Antiquities of Cornwall," p. 37. 



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