SHELLS. 59 



Carbonate of lime, 66 

 Membrane, . 24 



90* 



Pearl, a well-known globular concretion, which is formed in 

 some of these shells, resembles them exactly in its structure and 

 composition. It is a beautiful substance of a bluish-white colour, 

 iridescent, and brilliant. It is composed of concentric and alter- 

 nate coats of thin membrane and carbonate of lime. The iri- 

 descence is obviously the consequence of the lamellated struc- 

 ture.! 



It is said that the inhabitants of Ceylon have discovered a very 

 remarkable way of bleaching pearls that have become yellow. 

 They mix them with the seeds mingled with earth, with which 

 they feed their fowls ; the birds swallow the pearls ; the stomach 

 is opened in one or two minutes after, and the pearls are found 

 perfectly bleached. Were they left too long in the stomach, 

 they would doubtless be dissolved.^ If this statement be true, 

 might not the pearls be bleached by steeping them in a very di- 

 lute muriatic acid for a minute or two. 



Mr Hatchett found that what is called the bone of the cuttle 

 fish is exactly similar to mother-of-pearl shells in its composition. 



From the comparative analysis of shells and bones, Mr Hat- 

 chett was induced to compare them together, and has shown that 

 porcelaneous shells bear a striking resemblance to enamel of 

 teeth ; while mother-of-pearl shells bear the same resemblance 

 to the substance of teeth or bone ; with this difference, that in 

 enamel and bone the earthy salt is phosphate of lime, whereas in 

 shells it is pure carbonate of lime. 



* Merat-Guillot, Ann. de Chim. xxiv. 71. 



t Hatchett, Phil. Trans. 1799. \ Jour de Pharmacie, xi. 175. 



