8 BULLETIN 679, U. S. DEPARTMENT OF AGRICULTURE. 



MEASURING THE OPTICAL PROPERTIES OF AN ALKALOID. 



Before measuring the optical properties of any substance, its 

 purity must be established, for admixtures may modify some or all 

 of its constants to a marked degree. The medium from which it is 

 crystallized must also be taken into consideration, as some sub- 

 stances take up the liquids, in the form of ''solvent-of -crystalliza- 

 tion," and the crystallographic and optical characters may differ 

 widely from one solvent to another. For instance, the alkaloid 

 quinidine is recorded in the literature as crystalhzing from acetone 

 in the tetragonal system, being optically uniaxial, from methyl and 

 other alcohols in the rhombic system, with axial ratios near 0.8:1 :0.7 

 and with an optic axial angle (2 E) about 75°, and from benzene, 

 also rhombic, but with the axial ratios nearer 0.7:1:1.1 and an optic 

 axial angle of 85°. These differences are such that the crystals from 

 these several solvents would be identified crystaUographically or 

 optically as belonging to different chemical substances by an observer 

 not previously informed as to their character. Anyone desiring to 

 gain famiharity with the manipulation of the microscope, and with 

 the sort of effects upon which this method of identification of sub- 

 stances is based, may follow the directions outlined for the typical 

 case described in the next paragraph. 



Dissolve a 5-gram sample of cinchonine alkaloid in 100 cc of 

 boiling 95 per cent alcohol. Filter the solution and allow it to cool, 

 when the alkaloid is deposited on the walls of the vessel in small, 

 briUiant crystals. Take out a few crystals, dry, crush, and mount 

 them on a slide in potassium-mercuric iodid solution with a refrac- 

 tive index of about 1.61. Examined first with a low-power objective, 

 in ordinary light, cut down by the substage diaphragm, the material 

 shows plates and rods, many of them with hexagonal outlines. When 

 the lower' nicol is inserted, with its plane of vibration horizontal 

 (right and left) , aU of the grains with their long dimensions horizontal 

 stand out boldly, and light appears to pass into them as the tube is 

 raised, indicating their refractive index lengthwise to be higher than 

 that of the liquid. Many grains with their long dimensions vertical 

 show faint orange and blue borders, indicating that the refractive 

 index crosswise approximates that of the liquid, although, as this 

 index is the mean between the two lower ones of the substance, it 

 varies with the position in which each grain happens to Ue. When 

 both nicols are inserted and crossed, the diaphragm being now opened 

 to obtain as bright an illumination as possible, the majority of the 

 grains are white, or have hues of the fourth order, because of the 

 extremely strong double refraction, with several marginal color bands, 

 due to the wedge-shaped edges. A few grains show brilliant hues of 

 lower orders, because of extreme thinness or because they are so 



