726 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



them, with greater difficulty than those of a less viscid quality, which may more 

 properly be called size: this difference is to be observed, when muscle is boiled 

 with repeated and frequent changes of water. Gelatin thus obtained, whether in 

 the state of mucilage, size, or glue, when completely dried,- is affected by water 

 according to its degree of viscidity : for, when cold water is poured on dry muci- 

 lage, it dissolves it in a short time ; but if cold water be poured on those varieties 

 of gelatin which, when dissolved in a proper quantity of boiling water, would, by 

 cooling, form jellies more or less stiff", it acts on them in different degrees, not 

 so much by forming a complete solution, as by causing them to swell and become 

 soft ; so that, when a cake of glue has been steeped 3 or 4 days in cold water, 

 if it swells much without being dissolved, and, when taken out, recovers its 

 original figure and hardness, by drying, such glue is considered to be of the best 

 quality. 



I shall soon have occasion to notice, in another place, the effects of acids and 

 of alkalies on gelatin ; it will therefore here be sufficient to observe, that as it is 

 soluble in acids, so, if dry mucilage, dry size*, and dry glue, be steeped in nitric 

 acid diluted with 3 or 4 parts of water, they will be progressively dissolved, 

 according to the degree of viscidity by which they are separately distinguished. 

 When the solutions of these substances in water were examined by the tanning 

 principle, and by nitro-muriate of tin, I have found that animal mucilage is more 

 immediately affected by the latter than by the former ; while the solutions of size 

 and of glue are equally acted on by both. And when gold dissolved in nitro- 

 muriatic acid was added to the solutions of mucilage, size, and glue, the gold 

 was reduced to the metallic state in a few hours, not only on the surface, where 

 it formed a shining metallic pellicle, but also on the sides of the glass, which 

 were thinly coated with a deep yellow sediment, which, like leaf-gold, appeared 

 of a fine pale green, when held between the eye and the light. 



The animal mucilage which I chiefly employed in these experiments, was ob- 

 tained from the corallina officinalis, as I found it to be pure, and not partly modi- 

 fied into gelatin or animal jelly-}-. But Mr. Bouvier asserts, that he obtained the 

 latter substance^ ; and this appears to me very probable ; for mucilage may pre- 

 dominate in this coralline at one period, and gelatin or jelly at another, just as it 

 is found to be the case with other animal substances ; for it is known, that in 

 young animals mucilage is abundant, and becomes diminished as these increase in 

 growth and age. Hence there is every reason to conclude, that the substance 

 which in very young animals was at first mucilaginous, becomes progressively 



* Gelatin obtained from eel-skin, evaporated to dryness. f By this I mean, that the 



mucilage had not acquired the degree of viscidity requisite to form a gelatinous substance. The 

 expression which I have employed, is not therefore to be understood as alluding to any essential 

 difference in composition, but only to denote some variation in the degree of consistency; for the 

 whole may be comprehended under the term gelatin, of which, mucilage may be regarded as one 



extreme, and the strongest and most viscid glue as the other. 1 Annales de Chimie. torn. 8» 



p. 311.— Orig. 



