40 SUBSTANCES SOLUBLE IN ALCOHOL. 



tannin, such as other acids, or bitter principles, the amount of 

 which may be approximately determined by deducting the weight 

 of the organic matter contained in the copper precipitate from 

 that contained in the lead. Under these circumstances the weight 

 of the organic substances precipitated by copper sometimes re- 

 presents approximately the tannin contained in the material 

 ( 52, 80). It must, however, be admitted that the great differ- 

 ence in the tannins occurring in nature prevents such a result 

 being looked for in every case. 



'* 51. Reactions. The following reactions are common to all 

 tannins : they are precipitated from aqueous solution by gelatine, 

 by many albuminous substances, by acetate of lead and copper, 

 stannous chloride, etc. ; they reduce, at least when warm, alkaline 

 solution of copper as well as solutions of gold and silver salts; 

 they strike an inky or dark-green colour with ferroso-ferric salts 

 and transform skin into leather. Some tannins are precipitated 

 by mineral acids, by tartar emetic and by alkaloids, but it is 

 frequently observable that an alkaloid and tannin which occur 

 together in the same plant do not form an insoluble compound. 



For the microscopic detection of tannin the reaction with iron 

 salts may be made use of. Cells containing tannin are moreover 

 coloured reddish-brown with bichromate of potash, violet-red 

 with aniline and reddish or violet with dilute solution of chloride 

 of zinc and iodine. (See note to 249.) 



The great difference shown by the various tannins ( 159 et seq.) 

 makes it exceedingly difficult to give any general rules for their 

 estimation. Some of my pupils 1 have therefore at my instance 

 tasted the behaviour of the more important tannins to the re- 

 agents that have been recommended for their quantitative estima- 

 tion. Before I give a short resum6 of the results they hav( 

 obtained I should like to observe that, in my opinion, the esti 

 tion of the tannin in the alcoholic extract, prepared as I hav( 

 described, is preferable to the determination in the aqueous ex- 

 tract, provided of course that the material is very finely powder* 

 that the tannin is insoluble in ether free from alcohol, and that tl 

 alcoholic liquid has been evaporated under diminished pressure 



1 Compare Giinther, Pharm. Zeitschr. f. Kussland, Jg. 1870, pp. 161, 193, 

 225, and ' Beitrage zur Kenntniss der in Sumach, Myrobalanen etc. vorkom- 

 menden Gerbsauren,' Diss. Dorpat, 1871, and other Dorpat dissertations sub- 

 sequently referred to. 



