27G TRANSACTIONS OF THE [^MAT 31, 



" bulb of concussion," as it would be termed by archteologists. 

 This is produced wlienever a mass of crystal or agate receives a 

 sharp blow from being dropped on the floor, struck forcibly with 

 a mallet, or from other cause; a funnel-shaped flaw is produced, 

 the snu^ll end being at the surface of the crystal. If no further 

 blow be given, the flaw may still develop toward the interior of a 

 crystal. Bulbs of concussion can be seen in any agate mortar 

 that has been extensively used in the laboratory. Dozens have 

 been observed in a single mortar. In the breaking of flint, this 

 same structure is developed. 



It might be possible for a ball containing one or more of these 

 bulbs of concussion to be })olished on a sand-stone wlieel, as is 

 done at Oberstein, but the flaws are likely either to develop to- 

 ward the centre of the mass, or else flake off to one side and 

 ruin the entire crystal. 



To determine whether a crystal is perfectly spherical, it should 

 be put into a bath of mercury. The lightest part will always 

 come to the top. 



The Japanese occasionally deceive unsuspecting persons by 

 selling to them glass imitations of ciystal. A single glance 

 would be sufficient to detect the fraud, if the purchaser liad ever 

 seen a pure, pellucid crystal ball, since it is impossible to produce 

 such a large i)ieco of glass entirely colorless. If the imitation 

 ball is placed beside a genuine crystal, the difference is at once 

 apparent. 



The following is a table of the weights of crystal balls of dif- 

 ferent sizes: 



25 mm. or 1 inch, — — 14 



f5l " 15 — — — 



Most of the rock-crystal used to-day comes from Madagascar 

 and Brazil. Friedeberg, Salzburg, Zillerthal in the Ty^"0^j 

 Hungary, and Ceylon contribute more or less sparingly to the 

 general supply; North Carolina, California, and other American 

 localities furnish some material which is rarely used, owing to 

 the small size of the crystals and the ciieapness of the foreign 

 mineral. 



In 1735 the yield from the Cave of Zinkenstock, near the 

 Grimsel, was valued alone at £2,250, It is also found in small 

 quantities at Jochle Berg, in the same region. 



