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Ohcrvatlons on the Processes of Tanning. 61 



The substance to be examined as to its tanning power 



„.av be used u. the ^-"^^^X^^;- -^^^.^t^^a:; of 

 1 M-nt,. rif cfiarsc Dowder, oi in small riagmep . n. 

 a :>tatL or coarse puu ' „. ■ j;?soh'e its astnngent 



boiling water will be suHic.ent to o 



principles. ,,^,ue, or gelatine, must be poured into the 



1 he >^"'^^.^}Pusion° till the eflect of precipitation is at an end. 

 ^*'i''He"turbid liquor must then be passed through a piece of 

 blottiiio-paper, which has been betbre weighed. 



When the precipitate has been collected, and the paper 

 dried, the increase of its weight is determined ; and about 2. 

 of this increase of weight may be taken as the quantity of 

 tannin in the ounce of Ihe substance examined. 



WJicn solution of gelatine cannot be obtained, a solution 

 of albumen mav be used. It is made by agitating the white 

 of an cQg in a pint of cold wat(;r. It does not putrefy nearly 

 so readlfy as the solution of glue, and it may be employed 

 with equal advantage in experiments of comparison ; but the 

 composition of the precipitates it forms with tannin, has 

 not as yet been ascertained. 



The'tanninir principle in different vegetables is possessed 

 of the same general characters; but it often exists in them 

 in states of combination with other substances. 



In galls it is in union with the gallic acid. In sumach it 

 is mixed with saline matter, particularly sulphate of lime ; 

 and in the greater number of barks it is in combination with 

 mucilage, and different extractive and colouring matters. 



Leather tanned bv means of different astringent infu- 

 sions, differs considerably in its composition ; but it seldom 

 contains more than l-Sd of its weight of vegetable matter. 



Gallic acid, and saline matters in general, in cases when 

 tlu'v are combined with tannin, are not absorbed with it by 

 skin ; but they remain in their primitive forms. 



The leather made from iniusions of Aleppo galls, and 

 of sumach, is composed probably of pure tannin and the 

 matter of skin. Its colour is very pale, and the increase of 

 weight is greater than in most other cases. 



Extractive, or colouring matters, in cases when they exist 

 in astringent infusions, as in the instance of oak !)ark al- 

 ready mentioned, are wholly or partly absorbed with the 

 tannin by the skin. The leather from barks in general is 

 coltAu-ed, and contains different proportions of extractive 

 matter. 



Of all the substances that have been examined as to their 



tauning properties, catechu or terra jaj)onica, is that which 



1.3 rieh-rsl in the tanning principle. This subitcuice is the 



:; " extract 



