40 SUBSTANCES SOLUBLE IN ALCOHOL. 



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

 Avhich may be approximately determined by deducting the weight 

 of the organic matter contained in the copper precijDitate from 

 that contained in the lead. Under these circumstances the weight 

 of the organic substances precipitated by co]5per 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. Beactions. — 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 copj^r, 

 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 

 Avith aniline and reddish or violet Avith 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^ 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 resume of the results they have 

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

 tion of the tannin in the alcoholic extract, pre})ared as I have 

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

 tract, provided of course that the material is very finely powdered, 

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

 alcoholic liquid has been evaporated under diminished pressure 



1 Compare Günther, Pharm. Zeitschr. f. Eussland, Jg. 1870, pp. ICl, 193, 

 225, and ' Beitrüge zur Kcnntriiss der in Sumach, Myrobalanen etc. vorkom- 

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

 sequently referred to. 



