TEAKL ESSENCE 29 



plete precipitation that Wulff ascribes to the tendency of the picrate 

 to dissociate. 



The writer tried solutions of the guanin from pearl essence in 

 hydrochloric acid, followed by precipitation with an excess of am- 

 monia, but the results were far too low. It may be significant that 

 all these methods give too low results. It has already been mentioned 

 that guanin particles adsorb foreign substances strongly. Perhaps 

 the specimens assumed to be pure contained more impurity than 

 was supposed, and that the real cj[uan titles of guanin were approached 

 by the analytical methods used. 



Bethe (1895) dried a drop of pearl essence on a microscope slide 

 and stained it with a warm solution of methylene blue. The mem- 

 branous and other organic tissue impurities are stained, while the 

 crystals are not. Upon microscopic examination an estimate can 

 be made of the degree of freedom from this class of impurity. 



For practical examination of pearl essence the wi-iter has been 

 using the following methods : 



MICROSCOPIC EXAMINATION 



The information to be gained by microscopic examination of pearl 

 essence relates to (a) shape and size of particles, (b) color of crystals, 

 and (c) presence or absence of foreign matter. 



If the essence is an aqueous suspension, put a small dot of it on a 

 microscope slide with a glass rod, mix thoroughly with a drop of 

 glycerin, and cover. If it is in an acetone or amyl acetate suspension, 

 use tricresyl phosphate or other nonvolatile, transparent liquid of 

 low index of refraction, miscible with the suspension liquid. Cover 

 with cover glass. 



Examine first by transmitted light with 16 and 4 millimeter 

 objectives, with the diaphragm almost closed. Note color, if any, 

 of the particles. Then examine by reflected light or dark field 

 illumination. For this purpose artificial light is better than diffused 

 daylight. Tilt the stage and arrange a desk light to illuminate the 

 slide obliquely. Note (a) shape and size of crystals, (b) whether 

 they are entire or fragments, and (c) uniformity of size of particles. 

 A rough estimate may be made of the concentration of the essence 

 by preparing a definite dilution (1: 100 or 1: 1,000) and counting 

 the particles in the Fuchs-Rosenthal counting chamber, though the 

 writer has found it diflScult to get consistent results by this method. 



Make a thin smear of the specimen on a slide and dry. Stain five 

 minutes with an aqueous solution of methylene blue, rinse, dry, and 

 examine. Note any stained particles of epidermis or other foreign 

 matter, being careful to distinguish between these and spots of dye 

 that may bo occluded between the crystals. Make another stain 

 with Sudan III, which stains fat a reddish-yellow. 



PHYSICAL ANALYSIS OF THE SPECIMEN 



Weigh out about 10 grams of the sample (acetone, amyl acetate, or 

 lacquer vehicle) in a tared weighing bottle. Dry to constant weight 

 at moderate temperature (50° for acetone, 70° C. for amyl acetate). 

 Weigh again and record the loss of weight as volatile solvent. Dis- 

 solve in about 100 cubic centimeters of acetone and filter by suction 



