WINE 195

it must further be remembered that, in the preparation of the liquid, the
wine has been diluted to double its volume. If the extract has not been
determined, a preliminary test is made by adding to boiling Fehling's
solution, drop by drop from a graduated pipette, the filtered saccharine
liquid prepared as described above. The addition is continued and the
boiling maintained until the cuprous oxide formed assumes a bright colour
readily distinguished with practice. This point represents the completion
of the precipitation and may be checked by filtering a few drops of the
boiling liquid, acidifying with acetic acid and testing with potassium ferro-
cyanide. The quantity of the saccharine liquid used in this preliminary
test indicates how many times the liquid must be diluted to give a solution
suitable for the determination.

This dilution is effected by running the necessary volume of the sac-
charine liquid from a burette into a 100 c.c. flask and making up to the
mark with water. The liquid thus diluted is placed in a burette and various
tests, made with Fehling's solution and with different volumes of the liquid,
as already described (see Sugars, General Methods), until with two quantities
of the liquid differing by o-i c.c., in one case only is a trace of copper detect-
able in the filtrate.

The mean of these two last determinations gives the volume a c.c. of
the diluted saccharine liquid necessary to reduce completely 10 c.c. of
Fehling's solution diluted with 40 c.c. of water. If n represents the number
of volumes of dilute saccharine liquid obtained from i vol. of wine, the
reducing sugars g per litre of wine, calculated as invert sugar, are given
by the formula :

To ascertain if the reducing sugars found contain excess of levulose or
of glucose, the theoretical polarisation, on the assumption that the reduc-
ing sugars consist of invert sugar, ~nay be calculated by means of the
formula :

y _ _g (43-66-- 0-5 <)

274-19

where g is the quantity of reducing sugar per litre and t the temperature
at which the polarisation is determined. If the polarisation found, P, is
greater — or more dextro-rotatory — than the calculated value, there is
excess of dextrose ; but if less — or more laevo-rotatory— there is excess
of levulose.

The respective quantities of levulose / and glucose d per litre may also
be calculated by the formulae :

i

9-278 — 0-04^
d~g-i,
where P is the polarisation (with the proper sign) of the undiluted wine.
For the application of these formulae in the case when saccharose is
present, see later, 3*