44 
POPULAR SCIENCE REVIEW. 
in the proportion of ash, owing to the production of sulphate of 
potassium. Sulphate of calcium (plaster of Paris) and cream 
of tartar give rise to the production of tartate of calcium (in- 
soluble), sulphate of potassium, and sulphuric acid ; and in wines 
so treated the ash rises frequently to five parts and more per 
thousand, consisting almost entirely of sulphate of potassium. 
The ash of these wines is frequently free from carbonates and 
chlorides, because the small quantity of sulphuric acid produced, 
as above explained, expels the weaker and volatile acids in the 
course of evaporation and incineration. No wine is met with 
free from chlorides. 
Total dry Residue and Extractives . — If a quantity of wine 
he carefully evaporated on a water-bath, a greater or less pro- 
portion of dry residue is left which contains all those constituents 
not volatile at 100°C. The greater part of this residue usually 
consists of sugar, acids, mineral constituents, &c. ; but in all 
genuine wines there is always, in addition, a small proportion 
of substances the exact nature of which is not at present known, 
and therefore do not as yet admit of quantitative estimation. 
Accordingly, if from the total dry residue left all those sub- 
stances admitting of estimation are subtracted, a small amount 
will be left unaccounted for. This consists of what are generally 
termed extractives. No genuine wine is free from these ex- 
tractives ; they may therefore form a valuable feature for dis- 
tinguishing the true from the factitious article. In wines free 
from sugar these extractives frequently constitute the greater 
part of the total residue. 
In pure natural wines the total amount of dry residue varies 
generally between fifteen and thirty parts per thousand. In 
fortified wines this total residue frequently rises to sixty, eighty, 
or even more per thousand ; it ranges generally between twenty 
and sixty parts, a great proportion of the residue in these cases 
being sugar. 
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