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Mohr cc at 17.5°, the temperature of the quartz wedges, as well
as the polarizing temperature, being 17.5°. This confusion
has been still further increased by the more recent readoption
of the true cubic centimeter as a unit for practically all scientific
work (volumetric apparatus now being furnished by the manu-
facturers, graduated upon this basis) and by the fact that there
is frequently no indication upon the instrument as to what unit
has been used in working out the scale. And it may be well to
remark here that the all too general custom in industrial (and
some college) laboratories of using all commercial volumetric and
other apparatus, and even weights, without calibration leaves the
accuracy of much analytical work in a very questionable light.
The only way by which accuracy can be assured is by calibrating
the flasks, burettes, pipettes arid weights to bo used in this work
and by checking the saccharimeter scale against quartz plates
that have been tested by the Bureau of Standards or by other
competent standardizing bureaus.
The International Scale.—In 1900 the International Sugar
Commission recommended that the sugar scale be redefined,
basing the 100° point upon the true cubic contimeter and a
temperature of 20° 0. Introducing tho correction for the change
of volume unit, and of the specific rotation of sucrose, tho expan-
sion of the glass polarizing tube, quartz wedges and metal scale,
between 17.5° and 20°, the normal weight of sucrose becomes
26.000 gm. The International sugar scale is then to bo defined
as follows: "The graduation of the saccharimeter xhall he made at
20° C., 26 gm of sucrose dissolved in iDater and the volume made
up to 100 metric cc. All weighings are to he made in air with
brass weights, the completion of the volume and the polarization,
are to be made at 20° C. This will determine the 100° point."
In order to determine the per cent of sucrose in a material
of unknown purity is only necessary to weigh 26.000 gin of tho
sample, dissolve and dilute to 100 cc and then "polarissc*" in
a 200-mm tube. If the material were pure cane nugar tho read-
ing would be 100° International (100° I.). If it were 50 per cent
pure the reading would be 50° I. In general, then, degrees on
this scale indicate per cent of sucrose. Of course it is essential
that no other active substance shall be present in the solution
or that some method for accounting for these shall bo available.