PEARLS AND MOTHER-OF-PEARL. 127 



are such familiar objects, to all Londoners at least; and they 

 acquire fresh interest when it is remembered that every 

 particle of the material for their shells was brought to them 

 invisibly, perhaps from the Alps, perhaps from the English 

 hills and downs, and perhaps from countries far away. 



Shells are of two kinds ; some, like the foreign cowries, 

 are hard and compact as porcelain, while others, like the 

 oyster, are formed in layers, and are often covered with a 

 skin ; but all consist chiefly of carbonate of lime and 

 gelatine (the hard sorts containing much less gelatine) with 

 varying proportions of sulphate of lime, phosphate of lime, 

 carbonate of magnesia, and iron. Pearls, which are secreted 

 by some other bivalves, as well as the oyster and mussel, 

 consist also chiefly of carbonate of lime, and are, in fact, of 

 the same composition as the mother-of-pearl forming the 

 shell. This is arranged in concentric layers like the coats 

 of an onion, round some minute particle of refuse, such as 

 a grain of sand, which the animal has been unable to re- 

 move from its dwelling, and therefore covered up as the only 

 way of diminishing the annoyance. 



It is needless to specify the various uses to which 

 mother-of-pearl is put, from the making of shirt-buttons, 

 to the inlaying of papier m&che ; but whatever its quality 

 and colour, its history is always much the same, and so, 

 too, is that of the shell cameos, though their texture differs 

 from that of mother-of-pearl. 



Cameos are cut from univalves shells formed all in one 

 piece, like the cowry, limpet, periwinkle, &c., and consisting 

 of three layers of calcareous matter, which in some are all of 

 different colours. In 1875, 300 persons were employed in 



