ON THE UNION OF TAN AND JELLY. tf 



The infusion of galls, however prepared, seemed inade- Mr. Hatchett's 

 quate to the purpose of affording an accurate test for jelly, artlflcial tan 

 I thought therefore of employing the artificial tan disco- 

 vered by Mr. Hatchett, because, being a substance formed 

 by a specific chemical action, it may be supposed always 

 to possess the same chemical properties. It was accordingly 

 prepared by digesting powdered charcoal in nitric acid, and 

 the result coincided entirely with the description of Mr. 

 Hatchett; it was readily dissolved both in water and alco- 

 hol, it precipitated jelly from its solution, and also the nitro- 

 muriate of gold, the muriate of tin, the superacetate of 

 lead, and the oxisulphate of iron. All these properties 

 show its strong resemblance to the infusions obtained from 

 astringent vegetables. I was however disappointed in not 

 finding it to answer the purpose that I had in view. Al- 

 though the artificial tan very readily afforded a precipitate formed anim- 

 from a gelatinous solution, yet the jelly seemed to be only p ? r ^ ct P reci " ' 

 imperfectly thrown down, the fluid remained muddy after 

 the operation, and the precipitated matter could not be com- 

 pletely separated from it. This circumstance I found to 

 take place with different portions of the artificial tan, which 

 were each of them prepared with every attention to Mr. 

 Hatchett's directions ; and, I conceive, depends upon a owing to th« 

 quantity of undecomposed acid, which remains attached to P re,ence oi }"** 

 the tan, and which cannot be entirely removed from it. acid. 

 This excess of acid was always found in my experiments, 

 and must probably have existed in Mr. Hatchett's prepara- 

 tions, for he points out their property of reddening litmus 

 as one that is characteristic of them *. To whatever cause 

 we may ascribe it, it seemed to be a sufficient objection to 

 the use of this substance as a test for jelly. 



Catechu was next tried, but without any better success.. Catechu does 

 Independent of the difference which exists between dif- not answer - 

 ferent specimens of this substance, which is considerably 

 greater than what is found in the infusion of galls, I have 

 never met with any catechu which is entirely soluble in wa- 

 ter. In the different trials that I made to procure standard 



* Phil. Trans, for 1805, p. 215. 



solutions 



