VOL. LXXXIX.] PHILOSOPHICAL TRANSACTION'S. 56l 



In like manner, if the effects produced by fire and acid menstrua, on shells 

 composed of mother of pearl, and on the substance of teeth and bone, are com- 

 pared, a great similarity will be found; for, when exposed to a red heat, 1°. They 

 smoke much, and emit a smell of burnt cartilage, or horn. 2°. They become of 

 a dark grey or black colour. 3°. The animal coal thus formed is of difficult incinera- 

 tion. 4°. They retain much of their original figure; but the membranaceous 

 shells are subject to exfoliate.* 5°. These substances, pearl, mother of pearl, 

 tooth, and bone, when immersed in certain acids, part with their hardening or 

 ossifying substances, and then remain in the state of membrane or cartilage. 6°. 

 When previously burned, and afterwards dissolved in acids, a quantity of animal 

 coal is separated, according to the proportion of the gelatinous, membranaceous, 

 or cartilaginous substance, and according to the duration of the red heat. And 

 lastly, the acid solutions of these substances, by proper precipitants, afford carbo- 

 nate of lime, in the one case, and phosphate of lime principally, in the other, in a 

 proportion relative to the membrane or cartilage with which, or on which, the one 

 or the other had been mixed, or deposited. 



As porcellaneous shell principally differs from mother of pearl, only by a relative 

 proportion between the carbonate of lime and the gluten or membrane, in like 

 manner, the enamel appears only to be different from tooth or bone, by being 

 destitute of cartilage, and by being principally formed of phosphate of lime, 

 cemented by gluten. 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 original 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 have in a great measure convinced 

 me, that membranes and cartilages (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 constituent principle. I 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, mem- 

 brane, 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 1 of their 

 constituent principles, but only that it has become deposited and mixed with them, 

 in proportion to the tendency they may have to form modifications of bone; or 



* This is a natural consequence, arising from their structure. — Orig. 

 VOL. XVIII, 4 C 



