V] PEARLS 61 



the pearls of periostracum and taking pearls pos- 

 sessing* an outer covering of nacre, we have the 

 following analyses : 

 Harley. 



Calcium carbonate ... 91*72 7o 



Organic matter ... ... 5*94 



Water 2-23 



Loss 0*11 



Dubois. 



Calcium carbonate ... 91*59 7o 



Organic matter 3*83 



Water 3*97 



Residue 0*81 



An analysis of nacre from the Ceylon pearl oyster 

 shells gave the following (Roaf) : 



Calcium carbonate . . . 88*79 7o 



Calcium sulphate 4*93 



Organic matter 2*32 



Water 2*28 



Loss 1*68 



The calcium carbonate occurs in the form known 

 as aragonite. 



So far as hardness is concerned it may be said 

 that some pearls require a hammer to break them. 

 This property varies greatly with the structure of 

 the pearl. 



Moebius found that pearls were scratched by 



