180 On the different Articles used for Tanning Leather. [Sept. 



The description of the mode in which the crystals of the 

 acetate of copper were disposed on the half immersed plates is 

 a faithful representation of the appearances that presented them- 

 selves. The interval of bright copper between the apparent 

 level of the fluid and the lower edge of the crystals is, f con- 

 ceive, to be attributed to a thin film of the fluid which adhered 

 to the metal, while the defined zone of crystals and the arched 

 form which they assumed on the uncoated side of the plate, 

 would appear to be referrable to some modification of galvanic 

 action. To the same cause we must perhaps refer the bands of 

 prismatic colours which passed across the copper between the 

 two edges of the tin. But I am aware that these phenomena 

 require farther examination. J. Bostock. 



Article IV. 



On the best Method of chemically ascertaining the relative Values 

 of the different Articles used for Tanning Leather. 



(To the Editors of the Annals of Philosophy.) 

 GENTLEMEN, Ross, Aug. 17, 1824. 



I should feel much obliged to you for the communication, 

 through your Annals, of the best method that is known of 

 chemically ascertaining the relative values of the different barks 

 and other articles used for tanning leather, sufficiently accurate 

 to guide the practical tanner. Elm and larch barks have been 

 used of late years by some persons ; but their value in 

 regard to that of oak is very imperfectly understood. Vallonia, 

 a Mediterranean product, a large kind of acorn, has been used 

 to a considerable extent, especially for sole-leather ; and more 

 recently the bark of the cork-tree. By precipitating a solution 

 of the tanning matter of cork-bark, and another of oak bark, by 

 carbonate of potash, and then filtering and drying the precipi- 

 tates, I find their value to be nearly as 15 to 8. Sumac, com- 

 pared in tanning quality with oak bark, has been estimated as 

 5 to 2, but oak bark tannage is tougher. Two pounds of sumac 

 will make one pound of leather. Five pounds of such bark as 

 tanners in the country get are requisite for the same effect. 

 Two pounds of raw pelt are necessary to make one pound of 

 tanned leather. Dressing leather does not require so strong an 

 ooze, or infusion of bark, as sole-leather. What would be the 

 most economical method of extracting the tanning matter of 

 these substances? I am, Gentlemen, 



Your constant reader, F. C. 



