unds, easily soluble in water. Hence, they do
not serve for the separation or identification of the individual sugars.
But if the solution in which they are formed contains an excess of phenyl
hydrazine and is heated to the temperature of boiling water for some time,
the alcoholic group next to the aldehyde group (the terminal alcohol group
in ketoses) is first oxidized to an aldehyde and then a second molecule of
phenyl hydrazine is added on, as illustrated above, forming a
di-addition-product, known as an "osazone." The osazones are generally more
or less soluble in hot water, but on cooling they crystallize out in yellow
crystalline masses each with definite melting point and crystalline form.
All sugars which have active aldehyde groups in the molecule form osazones.
These afford excellent means of identification of unknown sugars, or of
distinguishing between sugars of different origin and type.

Glucose, mannose, and fructose all form identical osazones. This is because
the structure of these three sugars is identical except for the arrangement
within the two groups at the aldehyde end of the molecule (see formulas on
page 44). Since it is to these two groups that the phenyl hydrazine residue
attaches itself, it follows that the resulting osazones must be identical
in structure and properties. All other reducing sugars yield osazones of
different physical properties.

When an osazone is decomposed by boiling with strong acids, the phenyl
hydrazine groups break off, leaving a compound containing both an aldehyde
and a ketone group. Such compounds are known as "osones." The osones from
glucose, mannose, and fructose are identical. By carefully controlled
reduction, either one of the C==O groups of the osone may be changed to an
alcoholic group, producing thereby one of the original sugars again. Hence,
it is possible to start with one of these sugars, convert it into the osone
and then reduce this to another sugar, thereby accomplishing the
transformation of one sugar into another isomeric sugar.

=Formation of Glucosides.=--By treatment with a considerable variety of
different types of compounds, under proper conditions, it is possible to
replace one of the hydrogen atoms of the terminal alcoholic group of the
hexose sugars with the characteristic group of the other substance, forming
compounds known, respectively, as glucosides, fructosides, galactosides,
etc. The structural relation of methyl glucoside to glucose