1825.] comparativeTanning Powers of Astringents, 407 



is already so familiar to him, that I have great hopes it is caicu-' 

 lated to become generally useful. 



There can be no question of the correctness of the principle 

 of this plan, it being that in daily operation in every tannery, 

 yet the field is open for improvement, and the exercise of inge- 

 nuity in the conduct of it ; but having placed the subject within 

 the grasp of the manufacturer, I candidly confess his superior 

 right to prescribe the details, and, therefore, look up to him for 

 instruction in every thing connected with his handicraft opera- 

 tions. 



As, however, I have made several experiments to ascertain 

 the proper mode of proceeding, and acquired some experience 

 in the matter, I willingly communicate it, and devote the 

 remainder of this paper to hints which I hope may be of service 

 to the tanner in going through the test on his own account. 



As the object is to institute a comparison between two or 

 more astringents, and decide quickly on their respective merits, 

 whilst the articles are yet at market ; a few pieces should be 

 selected from each lot, so as fairly to represent every parcel. 

 The whole of each sample should be separately ground to pow- 

 der in a small coffee or pepper mill, and passed successively 

 through the same sieve, to place each in similar circumstances. 

 From these average samples, the operator may take equal 

 weights, and obtain complete infusions of each by agitating them 

 with successive portions of warm water till all the soluble matter 

 is extracted. 



Though boiling water will hasten the operation, it certainly tends 

 to decompose the astringent liquor afterwards, and induces it to 

 deposit a portion of insoluble matter which may interfere with 

 correct results. Water at blood heat (98° Fahr.) may be safely 

 applied ; bottles to infuse and shake the powders in, and a 

 piece of muslin to strain through, serve ihese purposes com- 

 pletely. Care must of course be taken to preserve and return 

 any powdered bark which may remain in the strainer, with the 

 next quantity of warm water. Successive additions in this 

 manner are exceedingly more powerful solvents than the whole 

 quantity applied at once. Their efficacy increases in a geome* 

 trical progression. 



When the several infusions yielded by one sample are united, 

 the average liquor will in general be found sufficiently weak to 

 be acted on by skin with the greatest effect; that is, to afford 

 all the colouring matter it contains along with the tan ; — an 

 advantage the tanner is prevented from obtaining in strong- 

 decoctions of bark. If his experience should lead him to think 

 a particular infusion too strong (which may occur in the exami- 

 nation of astringent extracts similar to kino, rhatany, and catechu), 

 he may add water to reduce it to what he would call a " safe 

 tanning strength." Aliquot parts of these infusions (one-sixth 



