40 SUBSTANCES SOLUBLE IN ALCOEOL. 



bannin, 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 approlximately the tannin contained in the material 

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

 ence in the tannins occui-ring in natmre prevents ■ such a result 

 being looked for in every case. 



§ 51. Beaciions. — The following reactions are common to all 

 tannins : they are precipitated from aqueous aolutioii 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 fonn an insoluble compoundi 



For the microscopic detedim-oi tannin the reactioii with iron 

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

 coloured reddish-brown with bichromate of ijotash, violet red 

 ■with axdline iand reddish or violet with dilute solution of chloride 

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



The' great difference shown by the various tamiins (§ 159 et seq.) 

 makes it exceedingly difficult to give any general rules for their 

 estimation. Some of my pupils ^ have therefore at mj instance 

 tested the behaviour of the more tmportiant tannins to the re- 

 agents that have been recommended for theirqnantitative estima- 

 tion. Before I give a short rismifd 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, prepared 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 



^Compare Giinther, Fhann. Zeitschr. f. Ruasland, Jg. 1870, pp. 161, 193, 

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

 menden Gerbsaiiren,' Disa. Dorpat, 1871, and other Dorpat diaaertations sub- 

 bsquently referred to. 



