250 TANNINS AND [CH. XII 



used as tests for sugars ; and since they are liable to split 

 off glucoses under the action of acids, etc., it is not too 

 much to say that it is hardly ever possible to ascertain 

 certainly whether free glucoses were, or were not, origi- 

 nally present without first removing tannins. Micro- 

 chemical tests for reducing sugars are useless unless it 

 has first been shewn conclusively that tannins are absent ; 

 all tannins, not only those which are glucosides, readily 

 reduce Fehling's, etc., solutions. 



It may in some cases be of interest to ascertain 

 whether a given tannin is a glucoside or not, since there 

 can be little doubt that the glucose which can be split off 

 from such tannins is plastic material. This is not quite 

 so easy as might seem apparent, but a satisfactory and 

 fairly simple process is given on p. 17, by which this can 

 be accomplished. 



If it is required to compare the amounts of tannins in 

 two extracts, one of the modifications of the permanganate 

 process can be used — full details are given in Trimble, 

 Tannins, pp. 48 — 51, or Sutton, Volumetric Analysis. 



It is generally sufficient to ascertain whether tannins 

 are present, and then to determine which of the methods 

 given is best adapted for their removal. 



The addition of basic lead acetate will almost certainly 

 carry down the whole of the tannins, many glucosides, 

 and any proteids, etc. present, but the precipitate is very 

 liable to contain the glucoses as well, and it is not at all 

 easy to wash them out with alcohol or water. The use of 

 large quantities of water for washing the precipitate is 

 particularly to be avoided, as the precipitates of tannins 



