QUICKSANDS AND FIRE-STONES 67 



I consulted the great experimental physicist, my friend 

 Sir James Dewar, in his laboratory at the Royal 

 Institution. He told me that the late Professor Tyndal 

 used to exhibit the production of flame by the friction of 

 two pieces of quartz in his lectures on heat, but made 

 use of a very large and rough crystal of quartz (rock- 

 crystal) and rubbed its rough surface with another large 

 crystal. Tyndal's note on the subject in his lecture 

 programme was as follows (Juvenile Lectures on Heat, 

 1877-78): "When very hard substances are rubbed 

 together light is produced as well as heat." Sir James 

 Dewar kindly showed me the crystals used by Tyndal, 

 the larger was 1 6 inches long and 4 or 5 inches broad. 

 We repeated the experiment in the darkened lecture 

 room, and obtained splendid flashes. The same smell is 

 produced when rock-crystal is used as when flint or 

 quartz pebbles are rubbed together. All three are the 

 same chemical body, namely, silica (oxide of silicon). 

 We also found that when the crystals were bathed with 

 water or (this is a new fact) with absolute alcohol, the 

 same flashing was produced by the friction of one against 

 the other. 



Later, with the kind assistance of Mr. Herbert Smith, 

 of the mineral department of the Natural History 

 Museum, I examined, with a spectroscope, the flash given 

 by two quartzite pebbles when rubbed together. No 

 distinctive lines or bands were seen ; only a " continuous " 

 spectrum, showing that the temperature produced was not 

 high enough to volatilize the silicon. I also examined 

 some pebbles of another very hard substance — nearly as 

 hard as silica (rock-crystal, quartz, and flint). This was 

 what is called " corundum," the massive form of " emery 

 powder " (oxide of aluminium). By grinding two of 

 these corundum pebbles with very great pressure one 



