and formation of Pearls. 277 



son's abridgment of the Transactions, and we must add the 

 strange annotation which they have made upon it. 



" This Mr Arnoldi had a very erroneous notion concerning 

 the formation of pearls, which are certainly not the eggs of cer- 

 tain muscles and other bivalves in which they are found. ,, 



OBSERVATIONS BY THE EDITOR. 



In the preceding very interesting paper, the origin of the 



seed pearl is, we think, well established. We are disposed to 



think, however, that Sir Everard is mistaken in ascribing the 



lustre peculiar to the pearl, and which cannot be given to arti- 



Jicial ones, to the nacral shining lining of the central cell. 



What Sir Everard means by the nacral shining lining of 

 the central cell can be nothing more than the inner surface of 

 the pearl when the nucleus is removed ; but that this has no- 

 thing whatever to do with the peculiar lustre of the pearl, may 

 be demonstrated by grinding down the flat side of a bisected 

 pearl till the surface of the central shell is entirely removed. It 

 will then be found that the pearl still retains its peculiar lustre. 

 We are persuaded, indeed, that if each successive annual growth 

 could be separated from the rest, each concentric sphere would 

 possess the pearly lustre, and become a perfect pearl. In proof 

 of this opinion I have succeeded in detaching one of the annual 

 films, and the surface of it is just as pearly and beautiful as the 

 whole pearl from which it was detached. 



It is not easy to give a precise scientific explanation of the 

 peculiar lustre of pearls. A pearly reflexion may be produced 

 by several very thin transparent films being put together, as 

 takes place in a piece of mica that has been strongly heated. A 

 similar effect takes place without heat in some minerals, such as 

 apophyllite and stilbite where there is one eminent cleavage, and 

 where the plates are partially separated. It is still more strikingly 

 seen in the oxidation upon very ancient glass, where the effect 

 is remarkably beautiful. In pearls, however, the light thrown 

 back to the eye is not reflected, as in the cases now mentioned, 

 merely from the surfaces of the combined films, but also at the 

 points of junction of the carbonate of lime and the animal mem- 

 brane of which the pearl is composed, the substance of the pearl 

 not being homogeneous like transparent films. The pink and 



