VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 720 



ferent membranaceous laminae, and as they exhibit the colour and lustre of mother 

 of pearl, it might be expected, that they should prove to be of a similar nature 

 with the substance of stratified shells, or in other terms, that they should consist 

 of membrane and carbonate of lime. But when scales perfectly clean, and sepa- 

 rated from the skin of different fish, such as the salmon and carp, had been im- 

 mersed during 4 or 5 hours in diluted nitric acid, till they became transparent, 

 and perfectly membranaceous ; the acid liquor, being then saturated with pure 

 ammonia, afforded a copious precipitate, which was proved to be phosphate of 

 lime. The spiculae of the shark's skin, formerly mentioned, were found to be of 

 a similar composition ; and we may therefore regard the spicula and scales of fish 

 as true bony substances, in which the membranaceous part is more predominant 

 than in common bone. I fully ascertained, that the phosphate of lime was 

 afforded by these substances only ; for when the different skins from which these 

 scales and spicula had been taken, were separately examined in the like manner, 

 no phosphate of lime was obtained. In addition to this I must observe, that the 

 silver or pearly hue of pearl, mother of pearl, and of fish-scales, is only assisted 

 and modified by the relative degrees of opacity produced, in mother of pearl and 

 in pearl, by the interposition of the particles of carbonate of lime, and in the 

 scales by phosphate of lime; for this peculiar lustre principally resides in the mem- 

 branaceous part, and remains with it when the acetous or muriatic acids are em- 

 ployed as menstrua, but is completely destroyed by the nitric acid. 



The horny scales of serpents, lizards, and such like animals, differ from the 

 foregoing ; as all of those which I have examined, consist merely of the mem- 

 branaceous or horny substance, in a more or less indurated state, and appear to 

 be devoid of phosphate of lime, as an ossifying matter. Horny scales in general, 

 and the scales of the manis pentadactyla may be mentioned as an example, afford 

 but very slight traces of gelatin after being long boiled in distilled water ; and this 

 small portion of gelatin can only be discovered by the tanning principle, and by 

 nitro-muriate of tin, unless a very large quantity of the scales has been employed. 

 Human nail digested in boiling distilled water during several days, was only soft- 

 ened ; and, like quill, afforded a slight cloud, by the addition of nitro-muriate of 

 tin. Shavings of ox's hoof, when long digested as above-mentioned, afforded a 

 liquor which, in like manner, was only made slightly turbid by nitro-muriate of 

 tin. Nail and hoof, when long boiled, became of a much darker colour. 



The horn-like crust which covers certain insects and other animals, was sub- 

 sequently examined; the experiments were principally made on the plates which 

 covered the body of a large African scorpion, and on the common tortoise-shell 

 of the shops. The plates taken from the scorpion were not apparently affected, 

 though digested for a long time in boiling distilled water. The tanning principle 

 produced no alteration, when added to the water; but a faint white cloud appeared, 

 on the addition of nitro-muriate of tin. Tortoise-shell, in thin slips, and shavings, 

 was digested in a similar manner during 3 weeks; but it was only slightly softened 



VOL. XVIII. 5 A 



