202 COLOURING MATTERS IN WINE. 



more deeply coloured. Here, then, we discover one 

 reason for the pale brown colour common to wines 

 which are not red. It is not necessary that the 

 grape juice should ferment in order that this ex- 

 tractive matter may oxidize and acquire a brown tint ; 

 on the contrary, its oxidation appears to be rather 

 checked than promoted by fermentation, as we learn 

 from the white grapes which have dried in the sun. 



Is any one acquainted with white raisins ? 



The extractive matter being oxidized, and from 

 colourless having become brown, a substance is formed 

 which, though then soluble in water and spirit, is 

 rendered insoluble by continued exposure to the air, 

 and resolved into a substance which is called by 

 Eerzelius deposit (absatz^or .apot^SBMIi^-wu^,,.. 



This body is of a humus-like nature, possessing 

 other qualities in common with humic acid, besides 

 that of dissolving with a brown colour in alkali, 

 or carbonated alkali, from which it may again be 

 precipitated by means of acids. It is sparingly solu- 

 ble in alcohol. 



The substance described above being always con- 

 tained in fermented grape juice, necessarily renders it 

 darker in the course of time, especially if free access 

 be allowed to the air, and this is the case when 

 wine is preserved in casks, and frequently drawn 

 off into others, as at every transfer it is saturated 

 with air. 



If the wine be evaporated by heat before fer- 



