XXXI 



IVORY 



401 



smaller the area of the secondary dentine. In the spot 

 ball, the liead is usually let into the "nerve/' 



(Jn one occasion a billiard ball received a smart 

 stroke from a cue ; to the astonishment (jf the player a 

 large core of secondary dentine as thick as a thumlj fell 

 out. This core traversed the complete axis of the ball. 



Transverse section through an elephant's 

 tusk showing its elliptical outline, 

 remains of tlie pulpchaniljer, the con- 

 centric rings and liark. 



The specimen is preserved in the museum of the Royal 

 College of Surgeons. 



The billiard liall is turned in the rough and kept in a 

 warm room at an eijuable temperature for two years : it is 

 then turned true. A well-seasoned billiard liall should 

 be a perfect sphere with a diameter of 2 j^^ inch, and 

 weigh five ounces. Its contour is always changing ; it 

 tentls to become ovoid. In addition to this inherent 

 change, an ivory ball contracts with cold and expands 

 with heat. During a match, expert players take care to 

 avoid the balls becoming chilled by currents of cold air, 

 as they are then irresponsive to fine strokes. 



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