VOL XC.] PHILOSOPHICAL TRANSACTIONS. 727 



more viscid, and assumes the characters of gelatin ; which, as animals increase in 

 age, is known to become more and more viscid, as has been already mentioned in 

 the foregoing pages. I am inclined therefore to consider mucilage as the most 

 attenuated, and as the lowest in order, among the modifications of gelatin. 



As the qualities of gelatin are so various, so the properties of the substances in 

 which it is present as a component part, are much influenced by it ; and when, 

 for example, the skins of different animals were compared, I have always found 

 that the most flexible skins afforded gelatin more easily, and of a less viscid qua- 

 lity, than those which were less flexible, and of a more horny consistency. The 

 skin of the eel possesses great flexibility ; and it affords gelatin very readily, and 

 in a large proportion. The skin of the shark also, which is commonly used by 

 cabinet-makers to polish their work, was in like manner, for the greater part, soon 

 dissolved, and formed a jelly, like the former. The epidermis or cuticle of these 

 skins, which is very thin and tender, though not soluble, was reduced into small 

 particles by violent ebullition, and the spicula on the shark's skin were also sepa- 

 rated. The skins of the hare, rabbit, calf, ox, and rhinoceros, were examined in 

 a similar manner, and with the like results ; but the gelatin obtained from the 

 hide of the rhinoceros, as far as the smallness of the piece of skin would allow 

 me to determine, appeared to be the strongest and most viscid. In every one of 

 these experiments, the true skin or cutis was principally affected, it being com- 

 pletely soluble, as Messrs. Chaptal and Seguin have well observed, by long 

 boiling; but that of the rhinoceros far exceeded the others in difficult solubility. 

 The cutis of these skins, when first boiled, swelled and appeared horny ; it was 

 then gradually dissolved ; but in the cutis of the rhinoceros a few small filaments 

 remained, which at length contracted and adhered to the cuticle. 



The cuticle of the different skins was softened, but not dissolved ; and, as the 

 cutis seems to be essentially formed of gelatin,* so the cuticle appears to contain 

 it, though but in a small proportion : it is however necessary to its flexibility ; for 

 when, after long boiling, the cuticle of these skins was dried, it became a brittle 

 substance, which was easily reduced to a powder. Hair was much less affected 

 than either of the above-mentioned substances ; and this, with others in some 

 measure similar, I shall now more particularly notice. The substances to which I 

 allude, are hair, feather, horn, horny scale, hoof, nail, and the horn-like crust 

 which covers some insects and other animals, such as the scorpion and the tor- 

 toise. These I shall now mention, in as concise a manner as the subject will 

 allow. When hair of various qualities, and taken from different animals, was 

 long digested or boiled with distilled water, it imparted to the water a small por- 

 tion of gelatin, which was precipitated by the tanning principle, and by nitro- 

 muriate of tin ; and when the hair had been thus deprived of gelatin, and was 

 subsequently dried in the air, the original flexibility and elasticity of it were found 



* The cartilages of the articulations are also completely soluble when Jong boiled with water; but 

 this by no means happens when other cartilages are thus treated. — Orig. 



