OPTICAL ROTATION

129

be passed through a solution of potassium dichromate "of such
concentration that the percentage content of the solution
multiplied by the length of the column of solution in centimeters
is equal to nine.^77
The Sugar Scale.—The simplest and most generally useful
scale for the polarimeter is the circular scale, divided into angular
degrees, with a vernier for greater accuracy in reading. But in
the practical use of the instrument for analytical purposes there
arises (as is usually the case when scientific instruments are
used for practical testing) a demand for a direct-reading scale
that can be interpreted in terms of the per cent of active sub-
stance, without calculations other than of the simplest sort. The
largest commercial use of the polarimeter is for sugar testing and
for this purpose there have come into general use three scale
systems: the Ventzke (German), the Laurent (French) and the
International, the latter being a development of the Ventzke
scale. A scale of one of these types is usually placed upon the
instrument, even when angular degrees also are indicated.
The Ventzke Scale and the Normal Weight.—In this system
a "normal^77 solution of cane sugar was first defined as one having
17 5°
a specific gravity of 1.100 at y^Vo-    Of course this is an entirely
arbitrary value but it served to fix the basis for the system. The
scale values were fixed by polarizing a solution of this concen-
tration in a 200-mm tube at 17.5°C., this defining the 100° point
on the scale. Because of the difficulties involved in preparing
solutions having this exact concentration by use of the hydrom-
eter alone, it became customary to make the normal solution
for fixing the scale points by weighing 26.048 gin of sucrose and
making the solution to 100 cc at 17.5°. This is the same as
Ventzke's solution. The "normal weight" was then 26.048
gm.
The adoption of the Mohr unit of volume (1 cc "Mohr"
= 1.00234 true cc) brought confusion into the scheme, as instru-
ment builders for a time used the old normal weight with the new
volume unit. The 100° point on the Ventzke scale was then
fixed^1 "by polarizing in a 200-mm tube a solution containing
26.048 gm of sucrose, weighed in air with brass weights, in 100
^1U. S. Bureau of Standards, Circ. 44, 27, 2nd ed. (1918).