"non-reducing." Advantage is
taken of this property for both the detection and quantitative
determination of the "reducing sugars." A standard alkaline copper solution
of definite strength, known as "Fehling's solution," is added to the
solution of the sugar to be tested and the mixture boiled, when the
characteristic brick-red precipitate appears. If certain standard
conditions of volume of solutions used, length of time of boiling, etc.,
are observed, the quantity of cuprous oxide precipitated bears a definite
ratio to the amount of sugar which is present, so that if the precipitate
be filtered off and weighed under proper conditions, the weight of sugar
present in the original solution can be calculated. The proper conditions
for carrying on such a determination and tables showing the amounts of the
various "reducing sugars" which correspond to the weight of cuprous oxide
found, are given in all standard text-books dealing with the analysis of
organic compounds.

=Fermentability.=--The common hexoses are all easily fermented by yeast,
forming alcohol and carbon dioxide, according to the equation

C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}.

The importance and biochemical significance of this reaction will be
considered in detail in connection with the discussions of the relation of
molecular configuration to biochemical properties (see page 56) and the
nature of enzyme action (see page 194).

=Formation of Hydrazones and Osazones.=--Another property of the hexoses
which is due to the presence of an aldehyde group in the molecule, is that
of forming addition products with phenyl hydrazine, known as "hydrazones"
and "osazones." For example, glucose reacts with phenyl hydrazine in acetic
acid solution, in two stages. The first, which takes place even in a cold
solution may be represented by the equation

C_{6}H_{12}O_{6}+C_{6}H_{5}.NH.NH_{2} = C_{6}H_{12}O_{5}:N.NH.C_{6}H_{5}
Glucose Phenyl-hydrazine Glucose-hydrazone + H_{2}O.

The structural relationships involved may be represented as follows:

CHO H_{2}N.NH CH=N-------NH
| /\ | /\
(CHOH)_{4} + | | = (CHOH)_{4} | |
| | | | | |
CH_{2}OH \/ CH_{2}OH \/

The hydrazones of the common sugars, with the exception of the one from
mannose, are colorless compo