528 PHILOSOPHICAL TRANSACTIONS. , . [ANNO 1799* 



saturating the infusion with the solution of glue, the whole of the tanning prin- 

 ciple may be separated by precipitation. 



For the gallic acid. — To the pound of bark left in the earthen vessels, and 

 already deprived of its tanning principle by these quick infusions, I added a given 

 quantity of water, to procure a strong infusion of the gallic acid, which requires a 

 longer time, say 48 hours. This infusion, when obtained pure,* affords little signs of 

 the presence of the tanning principle, when tried by the test of the solution of glue; 

 but, with the solution of sulphate of iron, it gives a strong black colour, the common 

 black dye, which differs in density, according to the quality of the bark: this may 

 be further proved by boiling a skain of worsted in the dye, by which the gradations 

 of colour will be very perceptibly demonstrated. Having thus obtained a point of 

 comparison; by making a similar infusion, under similar circumstances, of any 

 bark, or vegetable substance, and paying strict attention to the specific gravity of 

 the infusion, the quantity of precipitate of leather, and the density of colour pro- 

 duced by given quantities of one or the other test, the result will be, a compara- 

 tive statement of the respective powers of any bark, or vegetable substance. This 

 comparative statement I conceive to be sufficient for all commercial purposes. As 

 oak bark is the usual substance employed in the trade of tanning, if a quantity of 

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

 mercial utility of that bark or vegetable may be determined, by comparing its 

 quantity of tanning principle and price with those of oak bark. 



For an accurate chemical analysis, I have tried a variety of acids, and simple and 

 compound affinities; and having pursued the above experiments, at the same time 

 that I was employed on some in dying, I found the muriate of tin (the method 

 of using which is described by Mr. Proust in the Annales de Chimie), very conve- 

 nient. A solution of it, being added to the infusion of bark, forms a precipitate 

 with the tanning principle, leaving the gallic acid suspended: the precipitate is of a 

 fawn colour, and is composed of tanning principle and oxydated tin. By these 

 means, I have been enabled to form a comparative scale of barks ; which however 

 I do not produce as accurate. Oak bark, in its present state, as procured for com- 

 mercial purposes, differs very much in quality, from accidental circumstances: the 

 season of the year in which it is collected occasions a still more important differ- 

 ence, consequently the scale now produced must be very imperfect; but I am of 

 opinion, that by the pursuits of scientific men who may be inclined to investigate 

 this subject more fully, a very accurate scale may hereafter be formed. In the 

 following scale, I have taken Sumach as the most powerful in the comparative 

 statement; leaving however a few degrees, for a supposed maximum of tanning 

 principle, which I reckon 20. 



* It is hardly possible, from the intimate connection of the 2 principles, to separate them entirely by 

 infusion: in the infusion of tanning principle, there will always exist a little gallic acid; and, in an 

 infusion of gallic acid, a little tanning principle will commonly be present, unless the infusion of gallic 

 acid is very weak, and procured by a 3d or 4th watering.- -Orig. 



