40 EXPERIMETs'TS ON ASTRINGENT VEGETABLES. 



of weight of" the fkin is comparative!)' fm.all, yet it is rendered 

 perfediy infoluble in water ; and is found loft, and at the fame 

 time ftrong. 



The falurated aflringent infufions orbark"? contain much lefs 

 extradive matter, in proportion to their tannin, than the weak 

 infufions; andj when (kin is quickly tanned in them, common 

 experience fliows that it produces leather iefs durable than the 

 leather flovvly formed. 

 The common Befides, in the cafe of quick tanning by means of infufions 



opinion in favour ^f barks, a quantity of vegetable exlraaive matter is loft to the 



of what IS called ; ^ ■^ . . , 



feeling of the manufaclurer, which might have been made to enter into the 

 leather probably compofition of his leather. Thefe obfervations (liow, that there 

 is fome foundation for the vulgar opinion of workmen, con- 

 cerning what is technically called tha feeding of leather in the 

 flow method of tanning ; and, though the procefTes of the art 

 may in fome cafes be protraded for an unnecelfary length of 

 time, yet, in general, they appear to liave arrived, in con- 

 fequence of repeated practical experiments, at a degree of 

 perfeaion which cannot be very far extended by means of any 

 elucidations of theory that have as yet been made known. 



On the firft view it appears fingular that, in thofe cafes of 

 tanning where extradive matter forms a certain portion of the 

 leather, the increafe of weight is lefs than when the fkin is 

 combined with pure tannin ; but the faci is eafily accounted for^ 

 when we confider that the attradlion of fkin for tannin muft be 

 probably weakened by its union with exlra6live matter ; and, 

 whether we fuppofe that the tannin and extradive matter enter 

 together into combination with the matter of fl^in, or unite with 

 feparate portions of it, flill, in either cafe, the primary attradiou 

 of tannin for (kin muft be, to a certain extent, diminiflied 

 "Vegetables are In examining aftringent vegetables in relation to their powers 

 "f ''^'"''^"''^^"' of tannmg fl<in, it is neceffary to take into account, not only 



ring not merely ° ^ , -^ _ _ ' -' 



by the matter the quantity they contain of \\\'2fuh fiance precipitable by gela- 



thic glue can tine, but likewife the quantity, and the nature, of the extractive 

 precipitate. , . ^ ,. .^ ..,.., 



matter ; and, in cafes or companion, it is eflential to employ 



infufions of the fame degree of concentration. 

 Catechu is the It- is evident, from the experiments detailed in the Hid fec- 

 jnoO: powerful {\qx\, that of all the aftringent fubflances which have been as 

 'yet examined, catechu is that which contains the largeft pro- 

 portion of tannin ; and, in fuppofing, according to the common 

 eflimation, that from four to five pounds of common oak bark 



are 



