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LXXI. Observations on Catechuic Acid. By John Thomas 



Cooper, Esq.* 

 A SHORT time since I was requested to visit a tannery 

 r* where the principal tanning ingredient employed was 

 catechu, and among other matters my attention was directed 

 to a whitish substance which made its appearance on the ex- 

 ternal surface of the leather when the tanning process was 

 completed, and the uniform appearance of this substance over 

 the whole surface is considered by the proprietors as the test 

 of the perfection of their process of tanning, which is usually 

 accomplished in about fourteen days. The tanning liquor is 

 prepared by making an imperfect solution of the catechu in 

 warm water, or in the liquor that has been previously par- 

 tially exhausted of its tannin by a former operation ; the de- 

 pilated hides in their usual state are sewn up so as to form 

 water-tight bags, into which the tanning liquid, prepared as 

 above, is placed, so as to completely fill them ; they are then 

 placed on floors and turned once or twice a day into every 

 possible position to expose the hide as equally as possible to 

 the action and pressure of the tanning liquid, and as the pro- 

 cess of tanning advances the appearance of this white matter 

 becomes more and more evident, until at length it covers the 

 entire surface of the leather, and sometimes acquires consi- 

 derable thickness and solidity. In this state, however, it is 

 contaminated with many impurities, and after repeated trials 

 to obtain it in a state fit for examination, I found the follow- 

 ing simple method to answer the purpose I had in view very 

 well. The matter, as obtained by scraping from the surface of 

 the leather, was thrown on a filter of linen cloth and washed 

 with cold water until the water passed through very nearly 

 colourless ; by this means a quantity of tannin, mucilage, ex- 

 tractive matter, and a peculiar substance, which I have not 

 yet examined, were removed; the matter remaining on the 

 filter was then treated with hot water, either bv washing it 

 on the filter, or which is better, by removal into" a basin and 

 heating it with three times its bulk of water to near the boil- 

 ing point, when a brown-coloured solution was obtained, and 

 by filtering this while hot in a warm place, the substance which 

 has the characters of catechuic acid, catechine, or tanningenic 

 acid, is deposited as it cools, but the deposition of the whole I 

 find does not take place until many hours after it has become 

 cold, therefore, after a lapse of about twenty-four hours, it may 

 be thrown on a filter and washed with cold water, in which 



* Communicated by the Chemical Society; having been read December 

 18, 1843. 



