568 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



miles square there are 1,024,000 acres. Allowing for the annual increase 

 required, it is safe to say the hemlock forests will all he consunfed within 

 two hundred years. The object of the speaker was to direct attention to 

 the importance of introducing economies in the use of bark, and of finding 

 substitutes for it which can be used with success. 



The Chairman remarked that tanning embraced essentially two opera- 

 tions: 1st, the preparation of the skin; 2d, the application of tannin to it. 

 The oldest and most common method of removing the hair from the skin is 

 to place it in a vat containing a solution of lime, and to let it remain until 

 the epidermis or cuticle of the skin is so softened and loosened that it and 

 the hair can be removed very readily. The sulphide of calcium lias also 

 been used to effect this object. But the process adopted in the great tan- 

 neries of America is to hang the skins in a long, close room, where the air 

 is very moist, and kept between 44 and 56 deg. Fab. This low tempera- 

 ture is maintained by allowing a stream of cold water, usually spring 

 water at about 50* Fab., to pass around the ceiling of the room, and at cer- 

 tain points to be let out in fine jets, which have the effect of cooling the 

 air below the temperature of the water. At the expiration of about one 

 week the hair is loosened. By this method less of the skin is lost in the 

 removal of the hair, and it is said hides unhaired in this way make from 

 twenty to thirty per cent, more leather than where the limiiig process is 

 used. The object of the tanner being to increase the weight of the manu- 

 factured article, which is sold by the pound, "the sweating process," as it 

 is called, is certainly more profitable, but whether the leather thus made is 

 more valuable than that carried through the liming process, is not yet 

 determined. 



The application of the tanning liquor or ooze extracted from oak, hem- 

 lock, or other bark containing tannin, results in a chemical combination of 

 the tannin with the gelatin of the skin. Tannin is compospd of 54 equiva- 

 lents of carbon, 22 of hydrogen, and 34 of oxygen. Gelatin is composed 

 of 12 equivalents of carbon, 10 of hydrogen, 4 of oxygen and 2 of nitrogen. 

 It does not exist in a free state but can be extracted from skin by means 

 of boiling water. Gelatin swells up in cold water, may be dissolved by 

 hot water, but is insoluble in alcohol and etber. 



When the gelatin of the skin has combined with the tannic, or rather 

 gallo-tannic acid contained in the ooze, the process of tanning is completed, 

 and the product is leather. As it is interesting to know the relative value 

 of barks and vegetable products containing tannin, the following table is 

 presented, giving the percentage found in each substance mentioned; 



Kino 7d 



• Buteagiim 73 



Chinese nut giills 69 



Bark of sassafras root 58 



Bombay catechu '. 55 



Bengal catechu 44 



Terra japonica 40 



Alderbark 36 



Apricot bark ' 32 



Pomegranate iSS 



Cherry bark 24 



Oak bark from .14 to .20 



Hemlock bark from .14 to .20 



June berry IS 



Sicily Sumac 1(> 



t 



