50 EVENINGS AT THE MICEOSCOPE. 



depends, as originally shown by Sir D. Brewster) is 

 due, not to the outcropping of alternate layers of mem- 

 branous and calcareous matter, but to the disposition 

 of a single membranous layer in folds or plaits, which 

 lie more or less obliquely to the general surface." * 



Those beautiful objects, — so much prized for per- 

 sonal adornment, — pearls, are concretions accidentally 

 formed within the shells of such moUusks, and are 

 wholly composed of the inner layer. Drs. Kelaart 

 and Mobius have recently published some highly inter- 

 esting observations on the causes both of the irides- 

 cence and of the pearly lustre ; and these I will cite 

 from the abstract translation of them made by Mr. 

 Dallas. 



" The surface of pearls is not perfectly smooth, but 

 covered with very fine microscopic elevations and de- 

 pressions. These are more or less irregular in their al- 

 titude, but approach most nearly to equality in pearls 

 of the finest water. In pearls which exhibit a certain 

 iridescence, and which, when turned in diflerent direc- 

 tions towards the eye, present even very faint bluish, 

 greenish, and reddish tints, the surface is found to 

 present delicate irregular curved furrows, which either 

 run tolerably parallel to each other, or form small ir- 

 regular closed curves. This is due to the mode of 

 growth of the pearl, in which thin layers of nacre, of 

 small dimensions, have been laid over each other. 

 There is no continuous layer over the pearl, but a num- 

 ber of small portions which sometimes overlie the mar- 

 gins of the subjacent layers, and sometimes leave them 

 uncovered. This structure is seen most distinctly in 

 the pearl shell, where the conditions are rendered more 

 * Carpenter. The Microscope, p. 594. 



