2Z Experiments and Obfervations 



rnents, which beijig ufually employed for fuch purpofes, are 

 too xvell known to require defcription. 



It is neceffary moreover to obfcrve, that as the fubftances 

 examined were very numerous, sud my principal object was 

 to difoover the moft prominent characters in them, I did not 

 for the prefent attempt in general to afcertain minutely the 

 proportions fo much as the number and quality of their re- 

 fpcftive ingredients. 



The greater part, if not all, of marine (Iiells, appear to be 

 of two dcfcriptions in refpect to the fubllance of which they 

 are compofed. Thofe which will be firft noticed have a aor- 

 cellaneous afpecl with an enamelled furface, and when broken 

 are often, in a flight degree, of a fibrous texture. 



The {hells of the other divifion have generally, if not al- 

 ways, a ftrong epidermis, tmder which is the fliell, princi- 

 pally or entirely compofed of the fubftance called riacrc-, or 

 jnother-of-pearl. 



Of the porcellaneous fliells, various fpccies of voluta, cy- 

 pnea, and others of a fimilar nature were examined. 



Of the (hells compofed of nacre, or mother-of-pearl, I 

 ftlecled the oyfter, the river mulcle, the baliotis iris, and 

 the turbo olcarius. 



Experiinenfs on VorceUaneous Shells. 



Shells of this defcription, when expofed to a red heat in a 

 crucible during about a quarter of an hour, crackled and loft 

 the colours of their enamelled furface; thev did not emit 

 any apparent fmoke, nor any fniell like that of burnt horn 

 or cartilage. Their figure remained unchanged, excepting a 

 few flaws; and they became of an opaque white, tinged par- 

 tially with pale grey, but retained part of their original glofs. 



The fliells which had not been expofed to fire (whether 

 entire or in powder) diifolved with great eflervefcence in the 

 various acids ; and the folution afterwards remained colour- 

 lefs and tranfparent. But the fliells which had been burned, 

 upon being diflbl -ed, depofited a very fmall quantity of ani- 

 mal coal ; and thereby the prefence of fome gluten was de- 

 noted, although the proportion was too fmall to be difcovered 

 in the folution of the fliells which had not been burned. 



The 



