tANNINft. 79 



erack. They are then put into a vat, containing a sour fluid, 

 generally prepared by allowing rye or barley to ferment in it, by 

 which they are softened, and their pores are opened, so tliat they 

 can more easily imbibe the tan-liquor in which (hey are afterwards 

 to be immersed. This part of the process is called raising, as the 

 hides are considerably swollen by it, and also requires particular 

 attention, for if too long continued, the skin is destroyed by under- 

 going putrefaction. 



Instead of tiiis part of the operation, which is sometimes difticult 

 to accomplish, owing to the state of the weather, the hides are 

 plunged into a fluid composed of sulphuric acid and water, in the 

 j)ro|)ortion of about a wine-pint of the former to fifty gallons of the 

 latter, and allowed to remain there till sufficiently softened and 

 thickened. 



The next stage of the process is the tanning, which consists 

 merely in soaking the hide in a solution of an astriiigetit vegetable, 

 and making this unite with the gelatin, by which it is rendered no 

 longer liable to undergo putrefaction, insoluble in water, and in a 

 great measure impervious to it. The astringent substance usually 

 employed, is oak bark, procured from the trees which are cut in 

 the spring, when the sap has risen into them. After being reduced 

 to coarse powder, it is put into pits with water, by which a solu- 

 tion of the astringent matter is procured, called an ooze. In this 

 the hides are immersed for several weeks, being frequently turned, 

 to expose the wiiole of them to the infusion, and allow it to pene- 

 trate them. From this pit they are put into others, the liquor be- 

 ing successively stronger, till it is completely saturated. Should 

 the hides be very thick, the last into which they are put contains 

 some of the powder of the bark in alternate layers with them, by 

 which, as the infusion becomes weaker by the substance combin- 

 ing with the gelatin, inore of it is taken up, and it is thus always 

 kept of proper strength. In this way the skins are allowed to re- 

 main, till the whole are converted into leather, which is known by 

 cutting a small piece from them, and observing its appearance. If 

 the process be completed, the cut edge is of a brownish color, but 

 if the tan has not penetrated it thoroughly, there is a white streak 

 in the centre ; of course they must be left in it, till the whole as- 

 sume the brown tinge. The time required for accomplishing this, 

 depends on the thickness of the hide. Calves-skins take from two 

 to four months, and the thick sole-leather hides, from tifteen to 

 twenty. When this process is completed, the hides are removed, 

 and laid again across the beam, where they are smoothed, and well 

 heat, to make them more solid, and also more flexible, after which 

 the) are hung up on beams in the drying-house, a buildmg into 

 which the air is freely admitted, where they remain till dry. 



A French Chemist of the name of Seguin, proposed what seem- 

 ed to him an improvement in the mode of preparing leather, btit 

 which has not been adopted in this country. It consists in making 

 quickly, solutions of the oak bark of diiferent strengths, and pass- 

 ing the hides through them, beginning with the weakest, aiid end- 



