OPTICAL-CRYSTALLOGRAPHIC methods of identification have been used heretofore chiefly in connection with minerals, although their applicability to synthetic substances has been pointed out by several recent writers. Dr. F. E. Wright, in particular, has made preliminary examinations of several alkaloids, and shown the probable value of this method. The present writer is working out this method in detail, and applying it to the varied problems o,f the food and drug analyst. The paper abstracted d.eals especially with the methods of work which have been found appropriate in connection with alkaloids and related substances.
A simple chemical or petrographic microscope, provided with revolving stage, Nicol prisms, Abb~ condenser, sub-stage diaphragm, selenite plate, etc., is used. The observations made are as follows: in ordinary light, color, crystal habit, angles, cleavage, and refractive indices are observed. For the determination of the indices the immersion method is used, and, as the alkaloids in general are soluble in the immersion liquids ordinarily applied to minerals, special liquids are required. It :has been found that solutions of potassium-mercuric iodide in diluted glycerine are satisfactory for many substances. The determinations are made by diminishing the light by the sub-stage diaphragm and bringing a grain lying in a definite crystallographic position into. focus ; then, on raising the microscope tube slightly, a-band of light will pass into that substance, crystal or liquid, having the higher index in that direction. Successive liquids are tried until no light moves in either direction, when the index of the grain is equal to. that o.f the liquid, which is known.
By inserting the crossed Nicol prisms observations may be made in parallel polarized light, comprising the extinction angle, * Communicated by the Chief of the Bureau.