204 A TEXT-BOOK OF BOTANY. 



Quite a number of tannins yield a reddish, amorphous substance 

 which precipitates out of the acid solution and is insoluble in 

 water, alcohol, and solutions of the alkalies. Derivatives of this 

 kind are obtained from the tannin of kino, krameria, etc. 



2. When tannins are fused with potassium or sodium hydrox- 

 ide several classes of products are formed, depending on the con- 

 stitution of the tannin. (A) Protocatechuic acid is formed not 

 only on the fusion of certain tannins, but may be prepared from 

 other plant substances, as vanillin, asafoetida, myrrh, etc. 

 Usually other substances are formed in the interaction, these 

 being either acetic acid or phloroglucinol. In this class are in- 

 cluded the most of the tannins, which on heating with dilute 

 acids yield either phlobaphenes or insoluble red substances. (B) 

 Pyrogallol, which is commercially prepared by the dry distillation 

 of gallic acid, is also formed from the glucosidal tannins which 

 yield crystallizable acids in acid solutions. 



3. Upon carefully heating tannins to a temperature of 190 

 to 200 C. they are decomposed and yield two distinct classes of 

 derivatives, being either (A) pyrocatechol (a diatomic phenol) 

 or (B) pyrogallol (a triatomic phenol). Both of these substances 

 are crystalline and may be sublimed unchanged. They are, 

 furthermore, both soluble in water, alcohol, and ether, and are 

 distinguished by giving very characteristic reactions with certain 

 reagents. Solutions of pyrocatechol are colored dark green with 

 ferric-alum and greenish with copper sulphate -f- ammonium 

 hydrate, or concentrated sulphuric acid. Pyrogallol is colored 

 bluish-black with ferric-alum, becoming green and finally brown ; 

 brownish with copper sulphate -f- ammonium hydrate, or sul- 

 phuric acid, and becoming violet with lime water, rapidly chang- 

 ing to brown. Pyrocatechol is formed from those tannins which 

 produce protocatechuic acids on fusion with potassium hydroxide 

 and phlobaphenes or insoluble red substances on treatment with 

 acids. Pyrogallol is formed on heating those tannins which also 

 yield pyrogallol on fusion with potassium hydroxide and yield 

 either gallic acid or ellagic acid on hydrolysis with acids. 



MICROCHEMISTRY OF TANNINS. Tannin ocurs as a constituent 

 of the cell-sap, and the cells containing it may be determined by 

 use of dilute solutions of methylene blue, as proposed by Pfeffer, 



