it
passes through a solution of the carbohydrate in question. The amount of
the rotation depends upon the nature of the carbohydrate, the concentration
of the solution, and the length of the column of solution through which the
ray of polarized light passes. But the same definite amount of the same
sugar, dissolved in the same volume of water, and placed in a tube of the
same length, will always cause the same angular deviation, or rotation, of
the plane in which the polarized light which passes through it is vibrated.
In other words, the same number of molecules of the optically active
substance in solution will always produce the same rotatory effect. This is
called the specific rotatory power of the substance in question. It is
expressed as the number of degrees of angular deviation of the plane of
polarized light caused by a column of the solution exactly 200 mm. in
length, the concentration of the solution being 100 grams of substance in
100 cc. at a temperature of 20 deg. C. Actual determinations of specific
rotatory power are usually made with solutions more dilute than this
standard, and the observed deviation multiplied by the proper factor to
determine the effect which would be produced by the solution of standard
concentration. If the direction of the deviation is to the right (i.e., in
the direction in which the hands of the clock move) it is spoken of as
"dextro" rotation and is indicated by the sign +, or the letter _d_; while
if in the opposite direction, it is called "levo" rotation and indicated by
the sign -, or the letter _l_. For example, the specific rotation of
ordinary glucose is +52.7 deg.; of fructose, -92 deg.; of sucrose, +66.5
deg.

=Reducing Action.=--All of the hexose sugars are active reducing agents.
This is because of the aldehyde group which they contain. Many of the
common heavy metals, when in alkaline solutions, are strongly reduced when
boiled with solutions of the hexose sugars. Alkaline copper solutions yield
a precipitate of red cuprous oxide; ammoniacal silver solutions give silver
mirrors; alkaline solutions of mercury salts are reduced to metallic
mercury, etc. Any sugar which contains a potentially active aldehyde group
will exhibit this reducing effect and is known as a "reducing sugar." In
some of the di- and tri-saccharides, the linkage of the hexose components
together is through the aldehyde group, in such a way that it loses its
reducing effect; such sugars are known as