194 On the quantities of tanning principle and 



Having thus obtained a point of cofnparifon by making a fimilar infufion under 

 •*milar circumflances of any bark or vegetable fubftance, and paying flrift attention to~ 

 the fpecific gravity of the infufion, the quantity of precipitate of leather, and the 

 denfity of colour produced by given quantities of one ortlie other teft, the refult will 

 be a comparative ftatement of the refpeftive powers of any bark or vegetable fubftance. 

 This comparative ftatement I conceive to be fufficient for all commercial purpofes. 



As oak bark is the ufual fubftance employed in tfte trade of tanning ; if a quantity of 

 tanning principle is found to be contained in any other bark or vegetable, the com- 

 msrcial utility of that bark or vegetable may be determined by comparing its quantity 

 «f tanning principle and price- with thofe of oak bark. 



For an accurate chemical analyiis I have tried a variety of acids and fimple and com- 

 pound affinities ; and have purfued the above experiments at the fame time that I was 

 employed on feme in dying, I found the muriate of tin (the method of i^fing which is 

 defcribed by Mr. Proufi, in the Annates de Chimie) very convenient. A folution of 

 \f. being added to the infufion of bark, forms a precipitate with the tanning principle, 

 leaving the gallic acid fufpended ; the precipitate is of a fawn colour ; and is compofed 

 of tanning principles and oxidated tin. 



By thefe means, I have been enabled to farm a comparative fc^le of barks ; whkh, 

 however, I do not produce as accurate. Oak bark in its prefent flate, as procured for 

 commercial purpofes, differs very much in quality from accidental circumftances ; the 

 feafon of the year in which it is collefled occafions a flill more important difference ; 

 confequently the fcale now produced muft be very imperfeft ; but I am of opinion that 

 by the purfuits of fcientific men, who may be inclined to inveftigate tkis fubjeft more 

 fully, a very accurate fcale may hereafter be found. 



In the following fcale I have taken fumach as the moft powerful in the comparative 

 ftatement ; leaving, however, a few degrees ior difuppofed maximum of tanning principle, 

 which I reckon 20. 



Scale of Barks. 



Bark of 



Elm* 



Oak ciu in winter 



Horfe chefnut 



Beech 



Gallic acid by 

 colour. 



7 

 8 

 6 



7 



Tanning principle Tanning principle (ingrains) 



by hydrometer. from half a pint of infufioa 



and an ounce- of folulioa 



of glue. 



2,1 



2,1 

 2,2 

 2,4 



28 



3» 



* The infufion of elm was fo loaded with mucilag* that it was wilh diflkulty I oiuld feparate tlie tanning 

 principle, or try the fpecific gravity. 



WiUow 



