VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 725 



body, by age, and by the degree of health of animals, and also that the nature 

 of these, and even that of the unorganised bodies, does not always merely depend 

 on the proportion of the constituent principles, but also on the degree and mode 

 of combination to which these principles are subjected ; I say that when all this, 

 and the plan of the present paper, are considered, I natter myself that I shall not 

 be censured as hasty or negligent, if at this time I prefer a comparison of the 

 chemical properties of the bodies in question, with those of other substances 

 which, though not elementary, may be regarded as primary animal compounds ; 

 and when the subject is viewed in its full extent, the mode which I have adopted 

 will perhaps be deemed that which is the most satisfactory. 



^ III. Observations on the Component Parts of Membrane. 



In relating the preceding experiments, 1 have had frequent occasion to remark, 

 that a quantity of that animal jelly which is more or less soluble in water, and 

 which is distinguished by the name of gelatin, was obtained from many of the 

 marine bodies, such as the sponges, the gorgoniae, and others ; but in the experi- 

 ments made expressly to investigate the composition of membrane, it still more 

 frequently occurred ; and though in many cases, either from the small quantity 

 of the body under examination, or from the very small portion of gelatin thus 

 obtained, I was obliged to content myself with ascertaining the presence of it, 

 by the test of the tanning principle, and by nitro-muriate of tin;* yet in other 

 experiments, when the solutions of gelatin were gradually reduced by evaporation, 

 I had opportunities of frequently observing the various degrees of viscidity and 

 tenacity which characterize mucilage, size-)~, and glue. The difference in the 

 viscidity and tenacity of the varieties of these substances, is evidently an inherent 

 quality, and not caused by the degree of mere inspissation : if this was the case, 

 mucilage, size, and glue, when dry, would be of an equal quality, which is how- 

 ever contrary to daily experience ; for the varieties of glue are not of equal tena- 

 city. And it is well known, that glue made from certain parts of animals, such 

 as the skin, is more tenacious, and of a better quality, than that which is made 

 in some places from feet and sinews. 



Also, when even the same part is employed, which has been taken from 2 

 animals of the same species, an evident difference is found, according to the com- 

 parative age of the animals ; for the best and strongest glue is always obtained 

 from the more aged animals, in whom the fibre is found to be the most coarse 

 and strong. But a longer continued boiling appears requisite in order to extract 

 it ; and the more viscid glues are obtained, from the substances which afford 



* Nitro-muriate of tin has been proposed as a test for the tanning principle ; and the experiments 

 contained in this paper prove, that it may also be employed with much utility, to ascertain the pre- 

 sence of gelatin, and of certain modifications of albumen. \ The term size is employed, 



throughout this paper, to denote that modification of gelatin which appears to be intermediate, between 

 mucilage and the most viscid and tenacious gelatinous substances or glues. The weaker kinds of glue 

 may therefore come under this denomination. — Orig. 



