VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 731 



gelatin; and as the most remarkable effects were produced by nitric acid, I shall 

 to that confine the present observations. The specific gravity of the nitric acid 

 which I employed in the whole of my experiments, was 1.38; and this acid was 

 diluted with 2, 3, or 4 measures of distilled water, according to the quality of the 

 substance under examination, and the intended time of immersion. But as an 

 acid too powerful would have frustrated my intentions, I commonly added the acid, 

 by degrees, and at long intervals, to the water in which the substance was im- 

 mersed; during which time if any nitrous gas was discharged, more water was 

 added, as this gas was a certain sign that the acid was not sufficiently diluted. 



Substances like the corallina officinalis, which contain a large quantity of animal 

 mucilage, or of the least viscid jelly, soon impart it to boiling water. In like 

 manner, when such substances were steeped in nitric acid diluted with about 3 

 measures of water, the mucilage was in a few hours completely dissolved, while 

 the membranaceous part remained untouched. Pure isinglass dissolved in the di- 

 lute nitric acid, formed a pale yellow liquor, which by evaporation became of a 

 deeper colour, and when nearly dry was suddenly reduced to a spongy coal. This 

 change was rapid; and was attended with a considerable effervescence, and a copious 

 discharge of nitrous gas, not unfrequently accompanied by sparks, and sometimes 

 flame; arising undoubtedly from nitrate of ammonia, which was formed towards 

 the end of the evaporation. The acid solutions of mucilage, isinglass, and pure 

 glue, were changed to a deeper yellow, when saturated by the absorbent earths, by 

 the alkalies, and epecially by pure ammonia. In such cases, little or no precipitate 

 was obtained from pure gelatinous substances; but some faint traces of phosphoric 

 acid were discovered in these solutions. 



The effects of the dilute nitric acid on the other various substances which have 

 been mentioned, resembled those now described, and kept pace exactly with those 

 of boiling water; for when they were immersed in equal quantities of the dilute 

 acid during a given time, the solution of the gelatin took place according to the 

 order observed in those substances, when water was employed. As an instance of 

 this, 2 pieces of skin, recently taken from an ox, were subjected to experiment, as 

 follows: one of the pieces was boiled in water, till the whole of the cutis was dis- 

 solved; after which, the cuticle remained, though very feeble in texture, while the 

 hair did not seem to have suffered any material alteration. The other piece was 

 steeped in nitric acid diluted with about 4 measures of distilled water. At the end 

 of 5 days the cutis was dissolved, and the cuticle was become of a loose and feeble 

 texture; but the hair had not suffered any apparent change, except that of being 

 slightly tinged with yellow. In both cases therefore the effects of boiling water 

 and of acid were similar, and were evidently most powerful on those parts which 

 were the most gelatinous. 



As water dissolves mucilage more speedily than size, and this last more readily 

 than strong viscid glue, so are the effects of very dilute nitric acid on the same 

 substances; and when equal quantities of dried mucilage, of eel-skin glue, and of 



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