dant in Europe, but likewise contains much tannin. Every species 

 of oak, however, does not supply us with bark of the same quality; 

 the white oak is inferior to the green oak which grows in the south, 

 while this in its turn yields in the value of its bark to that procured 

 from the roots of the kermes-bearing oak, which is employed in 

 southern climates for tanning strong leathers. But whatever kind 

 of bark be employed, it is previously ground down to powder. 

 The tan-pits are sometimes of a round, and at others of a square 

 form, dug out to a considerable depth in the earth, and lined with 

 wood or masonwork; their size being in proportion to the extent 

 of the works. The method of tanning is different in different 

 countries. 



According to calculation, from five to six pounds of tan is 

 required to each pound of strong leather; and one hundred weight 

 of hides yields from fifty-two to fifty-six pounds of leather. 



It appears that the operation of tanning is nothing more than 

 combining the tannin, or astringent principle with the gelatin, 

 which is the basis of the skin, and all the manipulations of the 

 art are directed to effect or facilitate this combination. We will 

 now detail another method chiefly taken from Mr. Davy's memoir 

 on the subject. 



After the skin has been cleaned, it is submitted to other opera- 

 tions before it is immersed in the tan liquor. According to Mr. 

 Davy's account of the practices of the art, the large and thick 

 hides which have undergone incipient putrefaction, are introduced 

 for a short time into a strong infusion of oak bark, and after this 

 they are acted on by water impregnated with a little sulphuric or 

 acetic acid; in consequence of which they become harder and 

 denser than before, and fitted after being tanned, for the purpose 

 of forming the stouter kinds of sole leather. The lighter and thinner 

 skins are treated in a different manner: they are macerated for 

 some days in a ley formed from the infusion of pigeons' dung in 

 water, which contains a little carbonate of ammonia; the skin is 

 thus deprived of its elasticity, and becomes more soft. 



The tanning liquor is prepared by infusing bruised oak bark 

 in water; and skins are tanned by being successively immersed 

 in such infusions, saturated in different degrees with the astringent 

 principles of the bark. The first leys in which they are immersed 

 are weak, but towards the completion of the process they are used 

 as strong as possible; and in preparing stout sole leather, the 

 skins are kept in an ooze, approaching to saturation, by means of 

 layers of oak bark. 



80 



