4 ON THE UNION OF TAN AND JEJ.LY. 



nature. If finely powdered galls be infused for two hour* 

 in 3 times their weight of boiling water, an infusion is 

 formed, which is generally transparent, of a deep brown 

 colour, and which contains about one tenth of its weight 

 of solid matter. But although this is the usual result of 

 the process, it is by no means constantly so. Frequently 

 the infusion will be thick and muddy, will not be rendered 

 clear by being passed through the filter, nor will it become 

 so after standing at rest for several days ; its colour also va- 

 ries considerably, the brown tinge existing in different 

 shades of intensity, and occasionally being exchauged for a 

 bottle green. The quantity of solid matter contained in 

 the fluid is seldom precisely the same in any two trials; al- 

 though it is generally about one tenth, yet I have occasion- 

 ally found it no more than one fourteenth. Although it 

 may appear at first view somewhat singular, that such dif- 

 ferent effects should be produced by the same substance; 

 yet, when we attend to the visible difference, that exists in 

 gall nuts, we shall easily conceive how these variations may 

 orhomogene- take place. The structure of galls appears to have been 

 ous in their \\tt\ G attended to, and they have generally been spoken of 

 as homogeneous bodies, before the accurate description of 

 their several parts, that is given by the Mr. Aikins in their 

 late valuable publication*. 

 An extract of As it appeared impossible to employ a recent infusion of 

 them does not gal]s for the stari< }ard fluid, I thought of evaporating the 

 infusion, and making use of a solution of the dried resi- 

 duum. But I found, that this residuum, although formed 

 from a perfectly transparent infusion, is not capable of be- 

 ing completely redissolved, owing to some change that has 

 been effected on one or more of its constituents, probably 

 the extract, by which it becomes no longer soluble in water. 

 This circumstance forms an insuperable objection to the 

 employment of the dried residuum as a standard, because 

 the quantity of matter, depending upon the variable pro- 

 portion of the soluble and insoluble part, or of the tan and 

 extract, will scarcely ever be found the same in any two 

 specimens upon which we may operate. 



* Aikins 1 Chem. Diet. Art. Gall nut. 



The 



