368 Mr. Hatchett's Experiments on Zoophytes, 



on dry mucilage, it dissolves it in a short time ; but, if cold water 

 is 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 three or four 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, are steeped in nitric acid 

 diluted with three or four parts of water, they will be progres- 

 sively dissolved, according to the degree of viscidity by which 

 they are separately distinguished. 



When the solutions of these substances in water were ex- 

 amined 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 upon by both. And, when gold dis- 

 solved 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. 



* Gelatin obtained from eel-skin, evaporated re dryness. 



