Polytechnic Association Proceedixqs. 757 



Now whereas, there is eleven per cent of dried astringency or 

 pure tauuic acid in well saved hemlock bark, and tanners are only 

 making an average of one hundred and fifty pounds weight of 

 leather with every cord of bark they use, the facts I have just 

 stated clearly show Avhere a large proportion of this missing tannic 

 acid Is thrown away. But we will have to thoroughly scrutinize 

 our system of leaching to discover where a still missing balance 

 goes to; for it is an astounding fact, that the carefully kept statis- 

 tics of large American tanneries show, that the average exhibit of 

 the quantity of the tannic acid put into their leather, does not 

 exceed five per cent, which is less than one-half of what the bark 

 contains; and it is a self-evident fact, that many tanners are, as it 

 were, burning the candle at both ends, by destroying some portions, 

 while they are leaving the beforementioned share after them, still 

 held in the tan-bark. 



Having thus explained to you the grounds of my faith, in the 

 belief that time and money can be profitably expended in making 

 improvements on the present modes of spending bark, I will par- 

 ticularize some familiar correlative facts: 



1. What tanners call spent tan, is commonly selected as an ingre 

 dient to stop leaks in the bottom of canals, as the settlement of its 

 finer particles in the crevices or leaks is found a sure preventive 

 against the water leaching or leaking through. 



2. The fine particles of wood ashes, as they are carried down by 

 the lye in leaching, become arrested in every fissure, till they form 

 an impenetrable stratum, through which no liquid can pass — a dif- 

 ficulty which is very improperly met by breaking holes through 

 with a crow-bar. 



3. And the impossibility of operating on Sicily sumac by the 

 tanner's manner of leaching bark, are all different instances of 

 the same cause, producing the same effect as that which causes the 

 clogging of tanners' leaches, the passage of the menstruum through 

 gutters within the mass, and thus finding passages around the 

 ground bark, and not through each piece, as it should. 



Now, on looking to natural phenomena, we find that all descend- 

 ing waters carry with them more or less impurities, increased in 

 quantity in a ratio with the increased force of the current, and 

 constantly clogging up its old, and creating new passages to escape 

 through; while in a leaching point of view, we can find in nature but 

 one grand remedy for all these difficulties; being a remedy strikingly 

 illustrative and suggestive to a tanner, when observing that the 



