and Observations on Shell and Bone. 329 
those parts which were coated or penetrated by lines of enamel, 
are diminished, in proportion to the thickness of the enamel 
which has been thus dissolved; but little or no diminution is 
observed in the tooth.* 
Mr. Hunter has noticed this ; and, speaking of enamel, says, 
u when soaked in a gentle acid, there appears no gristly or 
“ fleshy part with which the earthy part had been incor- 
“ porated/'-f 
Now, when the difference which lias been lately stated, 
between porcellaneous shell and mother of pearl, is considered, 
it is not possible to avoid the comparing of these to enamel and 
tooth. 
When porcellaneous shell, whole or in powder, is exposed to> 
the action of acids, it is completely dissolved, without leaving 
any residuum. 
Enamel is also completely dissolved, in the like manner. 
Porcellaneous shell and enamel, when burned, emit little or 
no smoke, nor scarcely any smell of burned horn, or cartilage. 
Their figure, after having been exposed to fire, is not mate- 
rially changed, except by cracking in some parts : their external 
gloss partly remains, and their colour at most becomes gray, very 
different from what happens to mother of pearl, or tooth. In 
their fracture they have a fibrous texture; and, in short, the only 
essential difference between them appears to be, that porcellane- 
ous shell consists of carbonate of lime, and enamel of phosphate 
of lime, each being cemented by a small portion of gluten, 
* I have also observed, that when raspings of enamel are put into diluted nitric oi 
muriatic acid, they are dissolved without any apparent residuum ; but, when raspings 
of tooth or bone are thus treated, portions of membrane or cartilage remain, corre- 
sponding to the size of the raspings. 
f Natural History of the Human Teeth, page 35. 
