Experiments and Ohfervations on Shell md Bom. 503 



The fe {bells, when deprived by an acid menftruum of their hardening fubftance, or 

 carbonate of lime, appear to be formed of various membranes, applied ftratum fuper 

 ftratum. 



Each membrane has a correfponding coat or cruft of carbonate of iime ; which is fo 

 fituated, that it is always between every two membranes, beginning with the epidermis, 

 and ending with the laft formed internal membrane. 



The animals which inhabit thefe ftratified {hells, Increafe their habitation by the add!-, 

 tion of a ftratum of carbonate of lime, fecured by a new membrane ; and as every addi- 

 tional ftratum exceeds in extent that which was previoufly formed, the ftiell becomes 

 ftronger in proportion as it is enlarged, and the growth and age of the animal becomes 

 denoted by the number of the ftrata which concur to form the Ihell. 



Although the halhtis iris, and the tiirho oleartus, are compofed of the true mother of 

 pearl, I was induced to repeat the foregoing experiments on fome detached pieces of 

 mother of pearl, fuch as are brought from China. 



Thefe experiments I need not defcribe, as the refults were precifely the fame. 



I muft, however, obferve, that the membranaceous or cartilaginous parts of thefe 

 fhells, as of the pieces of mother of pearl, retained the exaiSl: figure of the ftiell, or piece 

 which had been immerfcd in the acid menftruum ; and thefe membranaceous parts dif- 

 tin£Hy appeared to be compofed of fibres placed in a parallel diredlion, correfponding to 

 the configuration of the ftiell. 



The fame experiments were made on pearls; which proved to be fimilar in compofition 

 to the mother of pearl ; and fo far as their fize would enable me to difcern, they appeared 

 to be formed by concentric coats of membrane and carbonate of lime ; by this ftrudture 

 they much refemble the globular calcareous concretions found at Carlftiad, and other 

 places, called pifolitlies. 



The wavy appearance, and irrefdefcency of mother of pearl, and of pearl, are evidently 

 the efFedt of their lamellated ftrutture and femi-tranfparency j in which, in fome degree, 

 they are refembled by the lamellatcd ftone, called adularia. 



When the experiments on the porcellaneous fliells, and on thofe formed of mother of 

 pearl, are compared, it appears that the porcellaneous (hells arc compofed of carbonate of 

 lime, cemented by a very fmall portion of gluten ; and that mother of pearl, and pearl, do 

 not differ from thefe, except by a fmaller proportion of carbonate of lime; which inftead 

 of being fimply cemented by animal gluten, is intermixed with, and ferves to harden a 

 membranaceous or cartilaginous fubftance ; and this fubftance, even when deprived of the 

 carbonate of lime, ftill retains the figure of the fliell. 



But between thefe extremes, there will probably be found many gradations ; and theftr 

 we have the greater reafon to expedl, fro^m the example afforded by the patellae, which 

 have been lately mentioned. 



Some few experiments were made on certain land fliells; and in tlie common garden 

 £nail I thought that I dilcpvered fome traces ofphofphatc of lime; but as I did not find 



3 T a any 



