NERVOUS TISSUE 375 



brown plates and prisms of choline periodide begin to form and may be detected by 

 means of the microscope. Occasionally they are large enough to be visible to the 

 naked eye. They somewhat resemble crystals of hemin (see page 268). If the 

 slide be permitted to stand, thus allowing the fluid to evaporate, the crystals will 

 disappear and leave brown oily drops. They will reappear, however, upon the 

 addition of fresh iodine solution, v. Stanek claims that this choline compound 

 has the formula C 8 Hi 4 NOI.I 8 . 



(b) Rosenheim's Bismuth Test. Extract the fluid under examination with 

 absolute alcohol, evaporate, and reextract the residue. Repeat the extraction 

 several times. Dissolve the final residue in 2-3 c.c. of water and add a drop of 

 Kraut's reagent. 1 Choline is indicated by the appearance of a bright brick-red 

 precipitate. 



1 Dissolve 272 grams of potassium iodide in water and add 80 grams of bismuth sub- 

 nitrate dissolved in 200 grams of nitric acid (sp. gr. i .18). Permit the potassium nitrate to 

 crystallize out, then filter it off and make the filtrate up to i liter with water. 



