492 TAIflfING SUBSTANCES. 



matters yielded by various lichens collected in the neighbourhood of Edin- 

 burgh, &c. 



BAEKS rOE TANMNa. 



Let us now take a brief re\dew of fhe sources from whence tan- 

 ning materials may be obtained, which will also enable ns to form 

 a fair estimate of the prospect of future supplies. Only one medal 

 was awarded, at the G-reat Exhibition, for tanning substances, viz., 

 to Messrs. Curtis, Brothers (United Kingdom, No. 126), but 

 honorable mention was made of the following competitors : — One 

 from Tunis, one from Yan Diemen's Land, one from New Zealand, 

 one from Belgium, one from the Cape of Grood Hope, one from 

 Canada, and one from the United Kingdom. 



The substance from which pui'e tannin is most frequently obtained 

 for chemical purposes is nutgalls, for tannin constitutes above 

 40 per cent, of their weight. It may be procured also from 

 several other som^ces, such as oak, horse chestuut, sumach, and 

 cinchona barks, catechu, kino, &c. 



The basis of the skins of animals is composed of a substance to 

 which the name of gelatine is given. One of the properties of 

 this substance is, that when combined with tannin, it forms the 

 compound of tannate of gelatine, or leather, a substance which is 

 so useful to mankind. Prom time immemorial, the substance em- 

 ployed to fiumish the tannin to the hides of animals, in order to 

 convert them into leather, has been oak bark. But as the purpose 

 for which oaks are grown is their timber, and not their bark, the 

 supply of oak bark cannot be calculated upon, and this is, perhaps, 

 one of the causes why tanning as an art is in such a backward 

 state. 



The consumption of tannin required in the leather manufacture 

 may be estimated from the fact that more than 672,000 cwts. of 

 raw hides were imported in 1851, besides the hides of the cattle, 

 &c., consumed in the United Kingdom. On the Continent and in 

 the United States the consumption of bark for this purpose is also 

 considerable. 



The imports of bark for the use of tanners and dyers has 

 amounted yearly to the very large quantiy of 380,674 cwt., 

 besides what we obtain at home. Oak bark contains usually the 

 largest proportion of tannin, and according to Davy's experiments 

 eight-and-a-half pounds of oak bark are equivalent for tanning 

 purposes to t\vo-and-a-quarter of galls, three of sumach, seven-and- 

 a-half of Leicester willow, eleven of Spanish chesnut, eighteen of 

 elm, and twenty-one of common willow bark. Tannin obtained 

 from these sources, however, differs materially in some of its 

 characters. The tannin of nut-galls, which is that generally em- 

 ployed for chemical purposes, is sometimes called gallo-tannic acid, 

 to distinguish it from other species. 



