92 Drs. Kelaart and Mobius on the Natural History of 



cause is doubtless effective in heightening their lustre. The 

 large amount of organic matter contained in them also explains 

 their low specific gravity, which is 2-650-2-686, or 0* 1-0-3 less 

 than that of pure calcareous spar or Arragonite. 



The surface of pearls is not perfectly smooth, but covered 

 with very fine microscopic elevations and depressions. These 

 are more or less irregular in their altitude, but approach most 

 nearly to equality in pearls of the finest water. In pearls which 

 exhibit a certain iridescence, and which, when turned in dif- 

 ferent directions towards the eye, present even very faint bluish, 

 greenish, and reddish tints, the surface is found to present deli- 

 cate, irregularly curved furrows, which either run tolerably 

 parallel to each other, or form small, irregular, 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 all over the pearl, but a 

 number of small portions, which sometimes overlie the margins 

 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 simple by the layers being depo- 

 sited on a flat or but slightly curved surface. The distance of 

 the furrows from each other is not always the same : sometimes 

 they may be recognized with the simple lens, whilst on other 

 parts they approach within g^^^th of an inch of each other. 

 That the iridescence of nacre, or the nacreous colour, as distin- 

 guished from pearly lustre, is caused by the interference of the 

 light reflected from these furrows and the intervening edges of 

 the strata, is proved by the circumstance, ascertained by Brewster, 

 that impressions of mother-of-pearl taken in red or black sealing- 

 wax exhibit the same phsenomena of colour distinctly. In pearls, 

 in consequence of their spherical form, the different masses of 

 coloured light are so diffused, that they unite to form white 

 light ; and this takes place with the greater perfection in pro- 

 portion as the furrows are lost, and become converted into a 

 surface of fine elevations and depressions. 



For their lustre pearls are indebted to their being composed 

 of thin layers, which allow light to pass through them, whilst 

 the numerous layers lying one under the other, disperse and 

 reflect the light in such a manner that it returns and mixes with 

 that which is directly thrown back from the outer surface. It 

 is the cooperation of light reflected from the surface with light 

 dispersed and reflected in the interior, that gives rise to lustre ; for 

 this reason the knots of window-glass exhibit pearly lustre, and 

 the membranes of pearls deprived of their lime are almost as 



* " This discovery of the stratification solves the contradiction between 

 Brewster's and Carpenter's representations of the matter." (Mobius.) 



