68 DIVERSIONS OF A NATURALIST 



against the other (using much greater pressure than is 

 needful in the case of quartz), I obtained flashes of light. 

 It was not known previously that any pebbles except 

 those of silica would give flashes of light when rubbed 

 together. A smell resembling that given out by rubbed 

 quartz, but fainter, was observed. 



Those are the facts new to me and to many others 

 about this curious subject. The flashing under water 

 is a very remarkable thing. I cannot say that I am 

 yet satisfied as to the nature of the flash. A simple 

 explanation of the result obtained, when two dry pebbles 

 are rubbed together in the air, is that crushed particles 

 of the quartz or of the corundum are heated by the heavy 

 friction to the glowing point. But this does not accord 

 with the fact that submergence in a liquid does not 

 interfere with the flashing. The rise of temperature 

 would certainly be checked by the liquid. And the 

 curious smell produced is in no way explained. 



The breaking of crystals is in many instances known 

 to produce a flash of light. Thus a lump of loaf sugar 

 broken in the dark gives a faint flash of blue light, as 

 anyone can see for himself immediately on reading this. 

 White arsenic crystals also, when broken by shaking the 

 liquid in which they have formed, give out flashes of 

 light. Some rare specimens of diamond, when rubbed 

 in the dark with a chamois leather, glow brightly. The 

 well-known mineral called Derbyshire spar, " Blue John," 

 or fluoride of calcium, when heated to a point much 

 below that of a red-hot iron, " crackles " and glows 

 briefly with a greenish light. The crystals of phosphate 

 of lime, called apatite, and a number of other crystals 

 have this property. But there is no record of any 

 peculiar smell accompanying the flashes of light. It is 



