Читать книгу The Book of the Pearl. The history, art, science, and industry of the queen of gems онлайн

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Chemically considered, aside from the nucleus, the structure of pearls is identical in composition with that of the nacre of the shell in which they are formed. Analyses have shown that those from the fresh-water mussels of England and Scotland, and from the pearl-oysters of Australia and of Ceylon, have nearly identical composition in the proportion of about 5.94 per cent. of organic matter, 2.34 of water, and 91.72 per cent. of carbonate of lime.^[70] The specific gravity ranges from nearly 2 to about 2.75, increasing with the deposit of the nacreous coatings. The following summary by Von Hessling^[71] shows the results of certain determinations of specific gravity:

AuthoritySpecific GravityNoteMuschenbroet2.750at moderate temperatureBrisson2.684at 14° RéaumurMöbius2.6864 fine pearls, weighing 2.396 gms.Möbius2.65024 pearls, weighing 6.221 gms.Möbius2.33663 brown pearls from Mazatlan, weighing 4.849 gms.Voit2.72216Voit2.616Bavarian pearls, 3⅝ carats, finer qualityVoit2.724Bavarian pearls, 1¾ carats, very fineVoit2.578Bavarian pearls, gray, with some lusterVoit2.765Bavarian pearls, brown, ranking between good & blackVoit2.238Bavarian pearls, poor black pearls, impure

Cross section of an irregular pearl, magnified 80 diameters