80 OUTLINES OF PHYSICAL CHEMISTRY
through a certain angle to the right or to the left, depend- ing on the variety (tetartohedral) of the quartz used. The quartz, therefore, had rotated the plane of vibration of the polarised rays coming from the first nicol.
Other crystalline substances, such as sodium chlorate, cinnabar, &c., possess an analogous rotatory power to that of quartz. This power is also possessed by a great number of substances which may be studied in the liquid state or in solution, and these frequently belong to the category of natural products ; — vegetable acids, carbohydrates, gluco- sides, terpenes and camphors, alkaloids, albuminoids.
For the investigation of the optical activity of such substances or their solutions, they might be enclosed in a tube with transparent ends and interposed between two
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crossed nicols, and the angle measured through which it was necessary to rotate the analyser to re-establish the original darkness of the field. But it is difficult to judge maximum darkness, and such a simple apparatus would not give sufficiently accurate results.
A number of improved polarimeters have been devised, which differ from each other chiefly in the means adopted to render the observation easy and accurate. As a typical apparatus we shall here describe Laurent's half-shadow polarimeter.
This instrument, intended for use with monochromatic yellow light (sodium flame), consists essentially of the following parts (Fig. 18).
A thin plate of potassium bichromate B for filtering the light, and thus only allowing the yellow to pass through.
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