and Observations on Shell and Bone . 331 
The difference, in the latter case, seems to explain why the 
bones and teeth of animals fed on madder become red, when, at 
the same time, the like colour is not communicated to the enamel; 
for it appears probable, that the cartilages which form the ori- 
ginal structure of the teeth and bones, become the channels by 
which the tinging principle is communicated and diffused. 
These comparative experiments prove, that there is a great 
approximation in the nature of porcellaneous shell and the 
enamel of teeth, and also in that of mother of pearl and bone ; 
and, if a shell should be found composed of mother of pearl 
coated by the porcellaneous substance, it will resemble a tooth 
coated by the enamel, with the difference of carbonate being 
substituted for phosphate of lime. 
Some experiments on cartilaginous substances (which I in- 
tended to have inserted in this Paper, but which I am prevented 
from doing, as they are not as yet sufficiently advanced,) have 
in a great measure convinced me, that membranes and carti- 
lages ( whether destined to become bones by a natural process, 
as in young animals, or whether they become such by morbid 
ossification, as often happens in those which are aged, ) do not 
contain the ossifying substance, or phosphate of lime, as a con- 
stituent principle. X mean by this, that I believe the portion 
of phosphate of lime found in cartilaginous and horny sub- 
stances to be simply mixed as an extraneous matter; and that, 
when it is absent, membrane, cartilage, and horn, are most 
perfect and complete. 
The frequent presence of phosphate of lime in cartilaginous 
substances, is not a proof of its being one of their constituent 
principles, but only that it has become deposited and mixed 
with, them, in proportion to the tendency they may have to 
