CHEMICAL SCIENCE. 287 



tities of 10 grms. with Avarm water, and exhausted by means of a. dis- 

 placement apparatus, constructed of a tube 2 feet long, 1 inch wide, and 

 drawn out at the lower end, which is loosely stopped with cotton wool. 

 Some substances may be introduced dry into this apparatus, and exhausted 

 by warm or cold water. The extraction may likewise be facilitated by 

 the pressure of a column of water, applied by fitting a narrow glass tube 

 with a cork into the upper end. 



In most cases, the extraction is completed in one or two days. "When 

 the operation is properly conducted, the quantity of liquid extract amounts 

 to half a pound or .a pound. It is then treated with gelatine solution so 

 long as a precipitate is produced. A few drops of dilute hydrochloric acid 

 facilitate the separation of the coagiilum. 



The quantity to be taken for an experiment of substances rich in tannin, 

 such as galls, is about 0.5 or 1.0 gnn. A simple calculation gives the 

 percentage of tannin. 



The author states that he has adopted this method in repeated examina- 

 tions of tanning materials during the last ten years ; he has found the re- 

 sults tolerably constant, and, notwithstanding its apparent imperfection, 

 more trustworthy than any other yet known. 



He estimates the relative value of several substances of this kind as 

 fellows : 



Pine bark contains from 5 to 7 per cent, tannin. 

 Oak bark contains 9 " " 



Best oak bark contains 19 to 21 " " 



Galls nuts contain 30 to 33 " " 



Aleppo galls contain 60 to 66 " " 



Chinese galls contain 70 . " " 



These data at least admit of comparison with each other, and indicate 

 with tolerable certainty the respective value of these substances to the tan- 

 ner. This method of valuation is indeed based upon the assumption that 

 the same kind of tannin exists in all these substances. It is, however, 

 extremely probable that this is not the case ; but, at the same time, it may 

 fairly be assumed, that if different kinds of tannin combined under simi- 

 lar conditions with different quantities of gelatine, they will also combine 

 animal skins in the same relative proportions. If, therefore, this method 

 does not indicate the absolute percentage of tannin, it still gives the per- 

 centage value of the substance examined ; and it is precisely this which 

 the tanner requires. , 



It is another question, whether gelatine solution precipitates all the 

 substances of the tanning material which combine with the skin ; and it 

 therefore remains to be determined by experience whether such a method 

 of valuation is sufficient for the purpose of the tanner. 



