A SUBSTANCE RESEMBLING SHELL. 
51 
brittle enamel, pure white, and having a pearly lustre. A fragment moistened on the 
back of the hand gives a sensation of heat, as quicklime does when so treated. The 
substance, when thrown into dilute muriatic acid, is entirely dissolved ; the fluid is 
tinged yellow, and the effervescence produces a froth, like beer. When the acid is 
very much diluted, and a portion of the substance is suspended in it, the solution 
takes place gradually, minute flocculi of animal matter being separated, and floating- 
in the fluid. 
2. The internal deposit. — This is separable into extremely thin laminae, and these, 
when in small fragments, are hardly distinguishable from scales of brown mica, show- 
ing also the most beautiful play of colours. The action of heat produces the same 
effect as on the external deposit, except that the separated laminae are thinner. The 
action of muriatic acid is the same, but the yellow tinge is deeper, and the froth is 
more permanent, indicating a larger proportion of animal matter than in the other. 
The nacreous lustre is also much more conspicuous in this. 
Mr. Gray, in his paper on the Structure of the Shells of Molluscous Animals, ob- 
serves that the pearly or iridescent lustre appears to be confined to shells of the con- 
cretionary structure, which when broken exhibit a nearly uniform texture, but sepa- 
rate when heated into numerous thicker or thinner laminae ; and he adverts to the 
observation of Mr. Hatchett, that when they are digested in weak muriatic acid, 
the lime is dissolved, leaving a great number of thin plates of animal matter, which 
retain the original shape of the shell. He adds, “ This variety of structure is found 
to constitute the whole shell of the Anomice and Placunce, and to form the inner coat 
of those shells which have pearly insides, as the Turbines, Haliotides, Uniones, &c., as 
well as the laminar portion of the Pinnae and Mother-of-pearl shells 
Besides the laminated structure, there is, in the case of the Pinna and some other 
shells, a prismatic crystalline arrangement of the particles perpendicular to, and pass- 
ing uninterruptedly through, the laminae ; but I have not discovered such an arrange- 
ment in any portion of the incrustation, even when examined by the microscope. 
I felt very desirous that this singular deposit should be examined by Sir David 
Brewster ; the more especially as he had long since directed his attention to the 
peculiar structure of mother-of-pearl -f-. On showing him the specimens in my pos- 
session he observed, that it was one of the most remarkable artificial productions he 
had seen ; and he readily undertook to examine it carefully. He shortly afterwards 
sent me the particulars of that examination, which had afforded some curious and 
interesting results. Having subsequently visited Catrine, I procured more perfect 
specimens ; and I sent these to Sir David Brewster, in order to ascertain whether 
they might not afford something new, in addition to the results he had obtained from 
the fragments he had formerly examined. They did so, and I now subjoin the very 
interesting account which Sir David has given me of the properties he has discovered 
in this new substance. 
* Philosophical Transactions, 1833, p. 794. 
H 2 
t Id., 1814. 
