208 



THE MllCHANICAL FUNCTIONS. 



or iridescent appearance.* 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 particles 

 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 be- 

 lieved to be a peculiar 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 materials, 

 and have the same laminated structure as the mem- 

 branous shells ; being formed by very thin con- 

 centric plates of membrane and carbonate of lime, 



disposed alternately, and often 

 surrounding a central body, or 

 nucleus: but Sir David Brewster 

 has satisfactorily shown that the 

 iridescent colours exhibited by 

 these surfaces are wholly the ef- 

 fect of the parallel grooves con- 

 sequent upon the regularity of 

 arrangement in the successive 

 deposits of shell. | The appearance of these 

 grooves or striae when highly magnified is shown in 

 Fig. 106. J This iridescent property may be com- 

 municated to shell lac, sealing wax, gum Arabic, 



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

 the shells of the Haliotis, or Sea-ear, and of the Anodon, or fresli 

 water muscle. 



t Philosophical Transactions for 1814, p. 397. 



I See also a paper on this subject by Sir John Herschel in the 

 Edinburgh Philosophical Journal, ii. 114, from which the annexed 

 figure is taken. 



