A study of the bi-refringence of agar agar.

Abstract

Many gels when subjected to strain become anisotropic. A film of gelatin or colloid dried on glass is anisotropic with one optical axis normal to the surface of the film. 1 With the removal of the strain as by moistening the film, the anisotropy largely disappears. The writers undertook to determine whether or not in the case of agar agar this property is inherent in the colloid itself or exhibited by it only in the gel state. To this end anisotropy was looked for in agar that had been dried in the gel form and in agar that had been dried in the sol form.

To obtain dried agar gel for examination under the microscope, drops of agar solutions ranging up to one per cent were placed on glass slides and dried in a vacuum desiccator at room temperature.

To obtain agar preparations dried in the sol form, drops of the same solutions were dried in a thermostat at 35°. That the agar agar preparation used in this work exists in the sol form at this temperature appears from a study of the viscosity of the solutions as the temperature is lowered. At various temperatures above 34° there is very little change in the viscosity. At 34° the viscosity begins to increase at an extremely rapid rate. Gelation occurred between 34° and 30°.

Under the polarizing microscope the agar dried as a gel shows very strong bi-refringence. The preparations were decidedly anisotropic. With a selenite plate addition colors were obtained. With a plate of red of the first order placed between the nicols at an angle of 45° to the plane of the nicols, addition colors were obtained, blue when the strix were parallel to the optical axis and yellow when perpendicular.