238 ANNUAL OF SCIENTIFIC DISCOVERY. 



varies to some extent. Several specimens of different species which I had 

 an opportunity to test ranged between calcite and fluor-spar; none were so 

 hard as fluor. 



Chemical Composition. The cocoa-nut pearl consists of carbonate of lime, 

 with a very small proportion of organic matter; so little that it does not 

 blacken nor evolve any odor before the blowpipe. When the cai'bonate of 

 lime is removed by the slow action of very dilute acids, a transparent sub- 

 stance remains, of great tenuity, showing no structure under the microscope, 

 and incapable of preserving its foi'm. The chemical reactions obtained with 

 it indicate that the organic substance is an albuminous body, and not cellu- 

 lose, the basis of vegetable tissues in general. Since albuminous substances 

 occur in plants as well as in the animal kingdom, we cannot find, however, that 

 it is of animal origin. True pearls consist of carbonate of lime, with a con- 

 siderable amount of albuminous animal matter. When decalcified by dilute 

 acids, the organic residue retains the form and structure of the pearl; and in 

 the nacreous pearls, the characteristic iridescence also. 



Microscopical Characters. Thin sections examined under the microscope 

 show that the cocoa-nut pearl is composed of numerous regularly concentric 

 laminae, adhering pretty firmly together. These layers form groups differing 

 slightly in tint, and near the exterior are often exceedingly thin. The centre 

 is occupied by a semi-transparent mass resembling the surrounding layers. 

 No foreign nucleus was found. The general mass is made up of radiating 

 bands of crystalline fibres, inclined at different angles in contiguous bands. 

 In the outer layers, the crystalline structure becomes strongly marked with 

 rhombohedral cleavage. Probably the great hardness of this pearl depends 

 upon the peculiar crystalline arrangement, with a little organic matter bind- 

 ing the whole firmly together. 



Pearls exhibit two principal A-arieties of microscopic structure. The true 

 or nacreous pearl is formed of concentric laminae of nacre, and shows a finely 

 furrowed surface, and no radiating lines within. The markings of the 

 nacreous membrane, by which iridescence is produced, are faintly visible in 

 the sections as very fine undulated and dotted lines. In the second variety of 

 pearl, a prismatic cellular structure occurs. These pearls exhibit well-marked 

 radiating lines, as 'well as concentric layers. In many specimens of pearl, 

 both varieties of structure are found. The cocoa-nut pearl presents a general 

 resemblance in microscopic characters to the second variety, but differs essen- 

 tially in the details of structure, as is evident from the sections now exhibited 

 of pearls from pearl oysters and from fresh-water clams, showing the 

 nacreous and prismatic varieties, and combinations of both. 



I cannot find that any species of pearl or other concretion resembling this 

 has been described. Nor could I learn from our best botanical authorities 

 that any concretion is known to occur in the cocoa nut. The milk of this 

 nut contains, according to the reported analyses, a little phosphate and 

 malate of lime, but no carbonate; nor is the carbonate found in any part of 

 the nut. Possibly an analysis of the immature nut might give a different 

 result. The only concretions of vegetable origin which approach this in 

 composition and structure are the cystolithes found in the leaves of Urti- 

 caceae, and some other families of plants. These are minute bodies, showing 

 concentric lamination. But they consist of a matrix of successive layers of 

 cellulose, upon which crystalline masses of carbonate of lime are deposited 

 in a kind of efflorescence; a wholly different mode of formation. 



In the animal kingdom, several kinds of concretions besides pearls bear 



