VOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 555 



nor of any other combination of phosphoric acid. The carbonate of lime was after- 

 wards precipitated by carbonate of ammonia ; and from many experiments it ap- 

 peared, that porcellaneous shells consist of carbonate of lime, cemented by a very 

 small portion of animal gluten. 



Previous to the experiments on shells composed of nacre or mother of pearl, I 

 examined some patellae from Madeira. When these were exposed to a red heat in 

 a crucible, there was a perceptible smell, like that of horn, hair, or feathers. The 

 proportion of carbonic matter deposited by the subsequent solution, was more con- 

 siderable than that of the shells above-mentioned ; and the proportion of carbonate 

 of lime, relative to their weight, was less. When the recent shells were immersed 

 in very dilute nitric acid, the epidermis was separated, the whole of the carbonate 

 of lime was dissolved, and a gelatinous substance, nearly liquid, remained ; but 

 without retaining the figure of the shell, and without any fibrous appearance. These 

 shells evidently therefore contain a larger proportion of a more viscid gelatinous 

 substance than those before mentioned ; but the solution, separated from the gela- 

 tinous substance, afforded nothing but carbonate of lime. 



Experiments on shells composed of nacre, or mother of pearl.- — When the shell of 

 the common oyster was exposed to a red heat, the effects were the same as those 

 observed in the patellae, and the solution of the unburnt shell was similar, only the 

 gelatinous part was rather of a greater consistency. A species of the river muscle 

 was next subjected to experiment. This, when burned in a crucible, emitted much 

 smoke, with a strong smell of burnt cartilage or horn ; the shell throughout be- 

 came of a dark gray, and exfoliated. By solution in the acids, a large quantity of 

 carbonic matter was separated ; and much less of carbonate of lime was obtained, 

 from a given weight of the shell, than from those already mentioned. On im- 

 mersing an unburnt shell in dilute nitric acid, a rapid solution and effervescence at 

 first took place, but gradually became less, so that the disengagement of the car- 

 bonic acid gas was to be perceived only at intervals. At the end of 2 days, I found 

 nearly the whole of the carbonate of lime dissolved ; but a series of membranes, re- 

 taining the figure of the shell, remained, of which the epidermis constituted the 

 first. In the beginning, the carbonate of lime was readily dissolved, because the 

 acid menstruum had an easy access ; but after this, it had more difficulty to insinuate 

 itself between the different membranes, and of course the solution of the carbonate 

 of lime was slower. During the solution, the carbonic acid gas was entangled, and 

 retained in many places between the membranes, so as to give to the whole a 

 cellular appearance. 



The haliotis iris, and the turbo olearius, resembled this muscle, excepting that 

 their membranaceous parts were more compact and dense. These shells, when de- 

 prived by an acid menstruum of their hardening substance, or carbonate of lime, 

 appear to be formed of various membranes, applied stratum super stratum. Each 

 membrane has a corresponding coat or crust of carbonate of lime ; which is so 

 situated, that it is always between every 2 membranes, beginning with the epidermis, 



4b 2 



