412 COMPOUND PRODUCTS. CHAP. I* 



Bark, so celebrated as a febrifuge and antiseptic, 

 are supposed to depend upon the quantity and qua- 

 lity of its tannin. But it is also of special utility 

 in its application to the arts, in consequence of its 

 peculiar property of forming an insoluble compound 

 with gelatine, by means of which the hides of animals 

 are converted into leather, and on which the important 

 art of tanning wholly depends. The bark of the 

 Oak-tree, which contains tannin in great abundance, 

 is that which is most generally used by the tanner. 

 The hides to be tanned are prepared for the process 

 by steeping them in lime-water, and scraping off 

 the hair and cuticle. They are then soaked first in 

 weaker infusions, and afterwards in stronger infu- 

 sions of the bark, till at last they are completely 

 impregnated. This process requires a period of 

 from ten to eighteen months, if the hides are 

 thick ; and four or five pounds of bark are neces- 

 sary on an average, to form one pound of leather. 



SECTION X. 

 Bitter Principle. 



A peculiar THE taste of many vegetables, such as those 

 lce ' employed in medicine, is extremely bitter. The 

 Quassia of the shops, the roots of common Gentian, 

 the bark and wood of common Broom, the calyx 

 and floral leaves of the Hop, and the leaves and 

 flowers of Chamomile, may be quoted as examples. 



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