ON THE UNION OF TAN AND JELLY. 7 



Pavy*, that isinglass consists nearly of pure jelly. I am Isinglass varia- 



not disposed to question the general fact ; but I may men- able in the „ 

 , , , - , • • , proportion of 



tion, as the result ot my own experience, that this is not al- i ts gelatine, 



ways the case ; and that, even in a majority of instances, 

 the isinglass that is procured from the fhops will be found 

 to contain a consiierable proportion of insoluble matter, 

 which I conceive to be of the nature of coagulated albumen. 

 The proportion of the matter soluble in water, which I re- 

 gard as pure jelly, and of the insoluble part, is very various. 

 In one instance, where the isinglass was boiled with twenty 

 times its weight of water, the jelly that was formed, instead 

 of holding in solution 5 per cent of solid matter, was found 

 to contain not more than 3*8, and although the addition of 

 more water carried off some of the substance which had been 

 left at the first boiling, still more than Y V°f tne isinglass was j, ut t h; s . m 

 left, apparently incapable of farther solution. This djrfi- be separated. 

 culty is obviated by boiling isinglass in water, pouring off 

 the jelly, and evaporating it to dryness ; by which means a 

 substance is procured, that is always ready for experiments. 

 But as this operation is attended with some trouble, 1 wish 

 to substitute for it a solution of glue, according to the pro- 

 cess employed by Mr. Biggin. Glue is entirely soluble in 

 water, and therefore does not present the objection that at- 

 taches to isinglass, yet there are some circumstances, which 

 seem to render glue less eligible for the purpose of experi- 

 ments. From the mode in which glue is prepared it might G j ue conta j ns 

 be supposed, that it would contain a quantity of albuminous albumen, 

 matter ; and I was confirmed in this opinion by finding, that 

 a solution of it has a precipitate formed in it by being boiled 

 with the oximuriate of mercury)*. The quantity of muriate mv , r i at e «f 

 of soda that exists in glue must be considered as an im- sorfa > 

 purity, which may have some effect upon the combination 

 ef jelly and tan. A more important circumstance, however, 

 and one which appears to have been disregarded by those 

 who have employed glue as a t^st for tan is, that, as it is 

 usually prepared, it coutains a considerable proportion of 



* Hatchett, Phil. Trans. 1800; D»vy, Phi!. Trans. 1803. 

 t This circumsUnce had been noticed by Dr. Thomson, Chem. Vol. 

 V, p. 479. 



water. 



