38 
POPULAR SCIENCE REVIEW. 
wine, would scarcely be perceived, the sugar and spirit in the 
latter case masking the sour taste. This effect should never be 
lost sight of in judging of the relative acidity of natural and 
fortified wines. Good wines of either sort contain in reality 
pretty nearly the same amount of acid, but in the fortified wine the 
sour taste is, as above described, less prominent. It is, however, 
a great mistake to suppose that, because the acid is masked to 
the taste, that it is not present ; and any injurious effects that may 
be supposed to follow its use in the one case will most assuredly 
also follow in the other. 
The acids of wine may be conveniently divided into two 
classes : firstly, non-volatile or fixed acids, such as cannot be 
distilled over or expelled by evaporation without decomposition ; 
and, secondly, volatile acids, which can be distilled or expelled. 
The acids of the first class are all derived directly from the 
grape juice, but are present in the wine in a different proportion 
to that in which they were contained in the must ; for the 
salts of some of these acids (those of tartaric acid, for example) 
are less soluble in the wine than in the must , and are conse- 
quently precipitated during fermentation. The chief acids of 
this class are malic acid and tartaric acid, the former usually 
predominating. In pure natural wines, made of ripe grapes, 
tartaric acid is probably never absent ; but its amount varies 
considerably, though rarely or never equalling the amount of 
malic acid present. In some wines part of the tartaric acid is 
replaced by the closely allied racemic acid. Generally all the 
tartaric acid contained in a wine is present in the form of an 
acid potassium salt (cream of tartar) ; only one half of it con- 
tributes, therefore, to the acidity of the wine, one half being 
neutralised by the potassa. 
In all fortified wines, plastered wines, or wines otherwise 
subjected to artificial treatment lessening the acidity, there is 
considerably less tartaric acid ; indeed, in such wines it is 
frequently totally absent. 
The volatile acids have all been formed during or after fer- 
mentation ; they are produced by the action of oxygen on some 
of the constituents of the wine, representing, so to speak, the 
first beginnings of its ultimate destruction. In small quantity 
they are indispensable to the wine, being chiefly instrumental 
in the production of its flavour and bouquet. When present, 
however, in large quantity, they are very objectionable, affording 
undoubted evidence of maltreatment of the wine, and rendering 
it very liable to turn sour altogether, i.e. to become vinegar. 
Such wines, therefore, should be carefully avoided. Among 
the volatile acids present the acetic is found in greatest propor- 
tion. and is formed by the oxidation of the ethylic alcohol. 
Besides this acetic acid there are always small quantities 
