

\ ••■^ ">,v ^ 



STRUCTURE OF SHELLS. ' IQO 



ance.* This appearance is caused by the peculiar thinness, 

 transparency, and regularity of arrangement of the outer 

 layers of the membrane, which, In conjunction with the par- 

 ticles of carbonate of lime, enter into the formation of that 

 part of the surface of the shell. The surface, which has thus 

 acquired a pearly lustre, was formerly believed to be a pe- 

 culiar substance, and was dignified with the appellation of 

 mother of pearl, from the notion that was entertained of its 

 being the material of which pearls are formed. It is true, 

 indeed, that pearls are actually composed of the same mate- 

 rials, and have the same laminated structure as the mem- 

 branous shells; being formed by very thin concentric plates 

 of membrane and carbonate of lime, disposed alternately, 

 ^^^^^^106 and often surrounding a central body, 



pfi^': "'-^ or nucleus: but Sir David Brewster 



has satisfactorily shown that the Iri- 

 descent colours exhibited by these 

 ;|; surfaces are wholly the effect of the 

 parallel grooves consequent upon 

 -n^s^^ the regularity of arrangement in the 

 -'^'^ successive deposltes of shell. t The 

 appearance of these grooves or strise 

 w^hen highly magnified is shown in Fig. 106.:]: This Iride- 

 scent property may be communicated to shell lac, sealing 

 wax, gum Arabic, balsam of Tolu, or fusible metal, by taking 

 an accurate cast or impression of the surface of mother of 

 pearl with any one of these substances.§ 



Porcellaneous shells have a more uniform and compact 

 texture than those of the former class. The animal matter 



* Examples of this nacreous structure, as it is termed, occur in the shells 

 of the ffalioiis, or Sea-ear, and of the ^iiodon, or fresh water muscle. 



f Philosophical Transactions for 1814, p. 397. 



^ See a paper on this subject by Herschel in the Edinburgh Philosophical 

 Journal, ii. 114. 



§ When these shells decay and fall to pieces, they separate into numerous 

 thin scales of a pearly lustre. The fine scales thus obtained from tlxe Tia- 

 cuna, or window oyster, are employed by the Cliinese in their water-colour 

 drawings to produce the effect of silver. Some of this powder hiu> been 

 brought to England and used for this piu'pose. Sec Gra\-, Phil. Trans, for 

 1833. 



Vol. I. > 22 



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