60 THE ISOLATION AND [CH. 



lead salt was decomposed either by sulphuretted hydrogen or sulphuric 

 acid. Microscopic crystals of the sodium, potassium, and ammonio- 

 sodium and potassium salts were obtained by evaporating the pigment 

 with alkali in a drop of alcohol. Senier gave to the lead salt, as a result 

 of several analyses, the composition Pb 2 C 21 H 29 30 . 



Gautier (149), 1878. The colouring matter of wine. 



Gautier was of the opinion that the colouring matters are formed 

 from tannins which become red on oxidation. He also suggested that 

 homologous series of colouring matters exist, such as C 20 H 20 10 , C 21 H 20 10 , 

 etc., and that each variety of Vine contains one or more members of 

 the series. The method he employed for isolation was to decompose 

 the lead salts of the pigments with hydrochloric acid ether, and then 

 to take up with alcohol and to precipitate with water. It has been 

 shown later by Heise that Gautier's pigments were mixtures. 



Heise (167), 1889. The colouring matter of grapes. 



Heise maintains that there are two pigments present in the skins 

 of purple grapes. One, soluble in alcohol, which he termed B, is 

 chiefly responsible for the colour ; the other, almost insoluble in alcohol, 

 he termed A. In wine, the soluble pigment B is said to be converted 

 by oxidation into A, which is then precipitated from the wine. 



The method described for the isolation of the pigments is to preci- 

 pitate the extract of fresh skins with lead acetate. The mixture of 

 the lead salts is then decomposed with hydrochloric acid ether, washed 

 with ether, taken up with methyl alcohol and precipitated from the 

 alcohol solution by adding ether. The product is again taken up in 

 alcohol and poured into water. The precipitate so formed is a mixture 

 of the two pigments which can then be separated by digesting the dried 

 product with absolute alcohol in the cold. Heise devises two methods 

 for separating the pigments. (1) The mixture of the pigments already 

 isolated is treated as above with absolute alcohol ; B goes into solution, 

 whereas A is insoluble. (2) The lead salts of the isolated pigments 

 are treated with acetic acid ; the lead salt of B is soluble in that solvent ; 

 the salt of A insoluble. 



Pigment A is a brown-black substance, insoluble in absolute alcohol, 

 ether, water and acetic acid ; but in alcohol containing a trace of acid 

 it is soluble to a red solution. 



Pigment B is soluble in alcohol to a brownish solution which, on 

 addition of acid, becomes red with a tinge of violet. 



