OPTICAL ROTATION

131

In order to make a simple reading possible it is not necessary
to use the normal weight of sample or to polarize in a 200-mm
tube. Any simple fraction or multiple of these numbers may
be employed and due account taken in the calculation. Polariza-
tion tubes are provided, varying by even stages from 100 to
400 mm in length.
The sugar scale provides direct readings for other sugar's and
for other optically active substances, not sugars, by use of a
properly modified normal weight. Thus for laetoso (milk
sugar) [a]" = 52.53, instead of (56.5 as for sucrose. Therefore
it will require p0>-;j X 20 = 32.9 gin of lactose in each 100 ec to
o^&. *)«>
give a rotation of 100° on the sugar scale. 32.9 gm is then the
normal weight for lactose and if, for example, 32.9 gm of milk
were treated in sueh a manner as to obtain the clear serum and
this diluted to 100 cc and then polarized in a 200-mm tube,
degrees International would indicate directly the per cent of
lactose in the milk. This determination in described in the
section on Dairy Products, page 214, Part III.
The Laurent Scale. -This in constructed so that a quartz
plate 1 mm in thickness and cut so that its faces are perpendicu-
lar to the optical axis will give a rotation of 100° L. The normal
weight for this scale will then be such that when this quantity
of substance is dissolved in 100 ee and the solution is polarized
in a 200-mm tube it will give the name rotation as a quartz
plate of the above description. Because of small differences in the
specific rotation of quartz specimens there has been some uncer-
tainty regarding the normal weight. The value now accepted
for sucrose is 10.29 gm, dilution to 100 true ec being understood.
Both Ventzke and Laurent scales are falling into disuse, being
properly replaced by the more rational International scale.
The Common Sugars.—Sucrose, or cane sugar, is the principal
sugar of the juices of sugar cane, sorghum, beets and many
fruits. It may be converted into a mixture of equal parts of
dextrose and levulose by hydrolysis, induced by action of acids:
The difference between the molecules of dextrose? and levulose in
a structural one and this has a direct bearing upon the rotation