2o6 TANNINS 



gums, or other bodies likely to interfere. This may be done 

 by extraction according to Pelouze's method (p. 210), or, if 

 the tannin is to be extracted from an aqueous solution, by 

 agitating with ether to remove gallic acid and then saturating 

 the aqueous solution with common salt and shaking with ethyl 

 acetate, which extracts the tannin. The ethyl acetate is then 

 evaporated off, the last traces being expelled by the repeated 

 addition of small quantities of ether. 



Another method is to extract with alcohol and to evapo- 

 rate off the alcohol at as low a temperature as possible, and 

 then to take up the residue with a large volume of water 

 whereby the phlobaphenes are precipitated and may be filtered 

 off. The infusion is then precipitated with successive small 

 quantities of lead acetate ; the first and last portions are rejected 

 and the middle fraction after washing is suspended in water 

 and saturated with sulphuretted hydrogen. The precipitated 

 lead sulphide is filtered off, and the solution is warmed to 

 drive off excess of gas and then extracted with ethyl acetate. 



Thus purified the tannin or its washed lead salt is heated 

 to 100" for an hour or more in a sealed tube or boiled in a 

 flask under a reflux condenser with hydrochloric acid (2 per 

 cent). After cooling, the mixture is allowed to stand for some 

 time and is then filtered from any deposit which may have 

 formed. The filtrate is shaken with ether to remove gallic 

 acid and the aqueous solution boiled, neutralized with caustic 

 soda and precipitated with basic lead acetate to remove any 

 unchanged tannin or colouring matter ; the solution is again 

 filtered and any lead remaining in solution is removed by the 

 addition of dilute sulphuric acid, excess of acid being carefully 

 avoided. The solution is then neutralized and once more fil- 

 tered and the clear filtrate heated to boiling with Fehling's 

 solution when a red precipitate proves the presence of glucose. 



THE CLASSIFICATION OF TANNINS. 



With the present incomplete state of our knowledge con- 

 cerning the chemical constitution of the tannins, it is difficult to 

 make a proper chemical classification of these substances. 



According to Trimble* the tannins may be divided into 

 two main groups : — 



* Tnrnble; loc. cit., vol. ii. p. 132. 



