asing the temperature of the solution; at 87
deg. its specific rotatory power is reduced to -52.7 deg., exactly equal
to but in the opposite direction of the effect of glucose; hence, _invert
sugar_, which is a mixture of an equal number of molecules of glucose and
fructose, and which has a specific rotatory power of -19.4 deg. at 20
deg. C., becomes optically inactive at 82 deg. C.

=Sorbose= is the only other ketohexose which has any importance in plant
chemistry. It does not occur free in plants, but is the first oxidation
product from the hexatomic alcohol, sorbitol, which is present in the juice
of the berries of the mountain-ash. Sorbose is a crystalline solid, which
is not fermentable by yeast, but which otherwise closely resembles
fructose.

DISACCHARIDES

The disaccharides, having the formula C_{12}H_{22}O_{11}, may be regarded
as derived from the monosaccharides by the linking together of two hexose
groups with the dropping out of a molecule of water, in the same way that
many other organic compounds form such linkages. That this is a perfectly
correct conception, is shown by the fact that, when hydrolyzed, the
disaccharides break down into two hexose sugars, thus

C_{12}H_{22}O_{11} + H_{2}O = C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}.

With all known disaccharides, at least one of the hexoses obtained by
hydrolysis is glucose; hence all disaccharides may be regarded as
glucosides (C_{6}H_{12}O_{5}.R) in which the R is another hexose group.

Since hexoses have both alcoholic and aldehyde groups, and since either of
these types of groups may function in the linkage of the two hexoses to
form a disaccharide, it is possible for two hexoses, both of which are
reducing sugars to be linked together in three different ways: (1) through
an alcoholic group of each hexose, (2) through an alcoholic group of one
and the aldehyde group of the other, and (3) through the aldehyde group of
each hexose. Disaccharides linked in either of the first two ways will be
reducing sugars, since they still contain a potentially active aldehyde
group; but those of the third type will not be reducing sugars, since the
linkage through the aldehyde groups destroys their power of acting as
reducing agents. Examples of each of these three types of linkage are found
among the common disaccharides, as will be pointed out below.

The following table shows the general characteristics of the common
disacchari