TESTS FOR PURITY OF CEREBROSIDES 483 



mg. in a small beaker (0.5 X 4 cm.) in 2 drops of pyridine at about 37°C. 

 This is transferred by means of a capillary pipette to a warmed microscope 

 slide, covered with a cover slip, and allowed to cool slowly. Spherocrystals 

 separate out first along the edges, preventing the further evaporation of the 

 solution. Since the crystals have the same refractive index as pyridine, 

 they are barely visible under the microscope in ordinary light. However, 

 when observed in polarized light Avith the crossed Nicol prisms, they stand 

 out from the dark background and exhibit well-defined crosses. If the 

 selenite plate with red I filter is placed directly over the polarizer, so that 

 its axis lies diagonal to the polarization planes of the crossed Nicol prisms, 

 characteristic differences are noted between phrenosine and cerasine. 

 Against the red background, the crystals appear divided into quadrants; 

 2 opposite quadrants with the addition color blue are contrasted with, the 

 other 2 with the subtraction color yellow. The relative position of the 

 crossed axes of the spherocrystals and of the a-axis of the selenite plate in 

 phrenosine, i.e., the upper right and the lower left quadrants being blue, is 

 reversed in cerasine, in which the upper left and the lower right quadrants 

 are blue. The axis direction of the selenite plate described by Rosenheim*^- 

 is the 7-axis. 



In the case of phrenosine, the spherocrystals are frequently replaced by 

 dense tufts of crystals, probably made up of individual needles arranged 

 side by side, with a feathery appearance. The tufts in which the vertical 

 axis runs parallel to the a-axis of the selenite plate are yellow. 



In the case of nervone, the separation always takes the form of well- 

 developed tufts of needles, which become integrated like felt after a few 

 hours. The color arrangement is opposite to that observed in cerasine, 

 i.e., the needles with a vertical axis parallel to the a-axis are blue. 



According to Rosenheim, if the mixture is made up of various cerebro- 

 sides, the phrenosine form appears first on the test. A few hours later, 

 forms with the opposite optical characteristics appear, and frequently sur- 

 round and envelop the phrenosine. 



Rosenheim''-- has used the test with success. It is applicable onlj^ when 

 the substance to be tested is completely free from phosphorus. Experi- 

 ence has shoAMi that, even in this case, occasional irregularities occur. In a 

 few instances, forms characteristic of cerasine have been observed to pre- 

 dominate in preparations of cerebron which were not entirely pure but 

 which were free from phosphorus. In fact, the separation products con- 

 sisted exclusively of these cerasine forms at first, and the opposite form did 

 not appear until later. 



Another test is the determination of the iodine number, preferably by 

 the procedure of Rosenmund, Kuhnhenn et al.*-^ The iodine numbers are: 



*** K. W. Rosenmund, W. Kuhnhenn, D. V. Rosenberg-Gruszynsky, and H. Rosetti, 

 Z. Untersuch. Nahr. Genussm., 46, 154-159 (1923);. Chem. AbsL, 18, 477 (1924). 



