on Shell and Bone, ^ 



I muft, however, obferve, that the membranaceotls or car- 

 tilaginous parts of thefe fliells, as of the pieces of mother-of- 

 pead, retained the exad figure of the fl.ell or piece .hich 

 had been immerfed in the acid menftruum ; and thefe mem- 

 branaceous parts diftinaiy appeared to be compofed of fibres 

 placed in a parallel dircdion, correfponding to the confagura- 



lionof the flicll. , 



The fame experiments were made on pearls ; which proved 

 to be (imilar in compofition to the mother-of-pearl; and lo 

 far as their fize would enable me to difcern, they appeared 

 to be formed bv concentric coats of membrane and carbo- 

 nate of hme :' by this flrufture they much refemble the 

 globular calcareous concretions found at Carlfbad, and other 

 places, called fifolithcs. ^ . c i 



The wavy appearance and irradiancy of mother-o -pearl, 

 and of pearl, are evidently the effed of their lamellated ftruc 

 tureandfemi-tranfparency; in which, in fomc degree, they 

 are refembled by the lamellated ftone, called adulana. 



When the experiments on the porcellaneous (hells, and on 

 thofe formed of mother-of-pearl, are compared, it appears 

 that the porcellaneous ftells are compofed of carbonate ot 

 lime, cemented by a very foiall portion of gluten ; and that 

 mother-of-pearl, a^^d pearl, do not dlfler from the e, except 

 bv a fmaller proportion of carbonate of lime ; which, inllead 

 of beincr fimply cemented by animal gluten, is intermixed 

 v/ith, and ferves to harden a membranaceous or cartilagi- 

 nous fubftance; and this fubftance, even when deprived of 

 llie carbonate of lime, ftill retains the figure of the flicll. 



But between thefe extremes there will probably be found 

 many gradations; and thefe we have the greater reafon to 

 expeft, from the example afforded by the patellae, uhich 

 have been latelv menlioncd. 



Some few experiments were made on certain land-fliells; 

 and in the common garden-fnail I thought that I d.fcovered 

 fome traces of phofphate of lime; but as I did not find any 

 ia the helix mcmoraUs, it may bq doubted whether the pre- 

 fence of phofphate of lime fliould be confidered as a chemical 

 charadcr of land-fliells * . Extcrz- 



* Some expc-rimants which I h.ve lately inadc upon :he cuttlcbcne^of 



Vor. VI. 



