12 REPORT — 1844. 



sence of grooves on the most highly-polished surface is due to the wearing- 

 away of the edges of the animal laminae, whilst those of the hard calcareous 

 laminte stand out. If each line upon the nacreous surface, however, indi- 

 cates a distinct layer of shell-structure, a very thin section of mother-of-pearl 

 ought to contain many thousand such layers, in accordance with the number 

 of lines upon its surface. But when the nacre is treated with dilute acid, so 

 as to dissolve away its calcareous portion, this is found not to be the case. 

 The number of layers of membrane bears no proportion whatever to the 

 number of lines upon its surface ; and it is impossible therefore to imagine, 

 that the laminations indicated by these lines are so many distinct layers of 

 shell-structure. 



25. It is generally difficult to ascertain anything from the examination of 

 the decalcified membrane, as to its disposition in the nacreous structure ; since 

 the disengagement of carbonic acid more or less completely unfolds the plaits, 

 of which some indications remain in it (fig. 19) : but one shell affords us the 

 opportunity of examining the plaits in situ, and thus presents a clear demon- 

 stration of the real structure of naci-e. The shell I allude to is Haliotis spleri- 

 dens, in which, as Mr. Gray has remarked*, a considerable quantity of animal 

 matter intervenes between the layers of nacre. This is not disposed in spots, 

 however (as stated by Mr. Gray), but in the form of numerous jj/a^e* of a 

 horny substance, very like tortoise-shell in colour and aspect. As the sur- 

 faces of these plates usually follow the curvature of the shell, a plane sec- 

 tion will not pass through any one of them for any considerable distance, 

 and consequently its cut portion will appear as an insulated spot. If a piece 

 of this shell be submitted to the action of dilute acid, the calcareous por- 

 tion of the nacreous layers, which intervene between these plates and hold 

 them together, is dissolved away, and they readily separate. Each horny plate 

 is then seen to be covered on one side with the membranous residuum of the 

 nacre, whilst on the other it is bare, — this surface being applied, in the un- 

 altered shell, to the layer of nacre which adheres to the next plate. Only a 

 single layer of nacre-membrane exists between each pair of horny laminae, 

 and we have thus a most favourable opportunity of studying its disposition. 

 It is generally found that, when the horny plates fall asunder in the dilute 

 acid, some of them exhibit the nacre-membrane in an undisturbed condition, 

 and their surfaces then exhibit the iridescent lustre, although all the calcareous 

 matter has been removed from the structure. On looking at the surface with 

 reflected light, under a magnifying power of about 75 diameter, it is seen to 

 present a series of folds or plaits more or less regular (fig. 18) ; and the iri- 

 descent hues which these exhibit are of the most gorgeous description. If 

 the membrane be extended with a pair of needles, these plaits are unfolded, 

 and it covers a much larger surface than before ; but the iridescence is then 

 completely destroyed. 



26. I think it will be admitted that this is an experimentum cmcis, in regard 

 to the cause of the iridescence of nacre, demonstrating that the peculiar 

 lineation of its surface (on which the iridescence undoubtedly depends) is 

 due, not to the outcropping of alternate layers of membranous and calca- 

 reous matter, but to the disposition of a single membranous layer in folds or 

 jilaits, which lie more or less obliquely to the general surface ; so that their 

 edges present themselves as lines, at a greater or less distance from each 

 other, according to the direction in which the section traverses them. 



27. Besides the images described by Sir D.Brewster, another optical phse- 

 nomenon has been pointed out by Sir J. Herschel, as presented by mother-of- 

 pearl, when light is reflected from its surface. This he has aptly compared 



* Pliilosopliical Transactions, 1833. 



