COLOURING MATTEES IN WINE. 201 



It is obvious that the cause of the slight yellow 

 colour which may be discerned in such wines, is not 

 to be found in any particular substance which deserves 

 the name of colouring matter, for vegetable juices, 

 well or ill preserved, or heated, when in contact with 

 the air, never appear perfectly colourless. Those sub- 

 stances which impart a brown tinge to colourless 

 vegetable juices when allowed to evaporate, and with 

 which the chemist is still but little acquainted, can, 

 when brought into contact with the air, suffer a like 

 alteration at a common temperature. 



"We may therefore expect to find in colourless 

 wines, when they have been prepared from grape 

 juice alone, without any admixture of skins, a more or 

 less perceptible yellow colouring the result of the 

 slow oxidation of a substance known by Scheele as 

 soapy-matter, to which Yauquelin first gave the name 

 of extractive matter, since it constitutes the principal 

 part of medicinal extracts a substance spread over 

 the whole vegetable kingdom, and capable of assum- 

 ing, as De Saussure observed, a brown colour, by ab- 

 sorbing oxygen, and giving off carbonic acid, when 

 exposed to the action of air and heat. 



This substance is found not only in the juices of 

 the leaves and fruits, but in every part of vegetable 

 products. It becomes gradually browner and browner, 

 but in a diluted state is called yellow. It is this 

 substance which is present in grape juice, and when 

 brought into contact with the air becomes more and 



