MICROSCOPIC EXAMIXATIOX. 341 



to secure an absolutely dark field, this can be done with a good 

 instrument and a cardboard tube over the slide with a near 

 approach to completeness. Any marked illumination of the 

 field when the Nicol prisms are crossed will greatly impair the 

 delicacy of the test. 



Microscopic Examination after Treatment with Sol- 

 vents. — A. Zega has suggested the following process : — The 

 sample is melted and filtered into a test tube, which is kept for 

 two minutes in a boiling water-bath. By means of a hot pipette 

 1 c.c. is measured into a 50 c.c. stoppered tube containing 20 c.c. 

 of a mixture of 6 parts of ether, 4 of alcohol, and 1 of glacial 

 acetic acid. The whole is well shaken, and allowed to cool in 

 water at 15° or 18° C. Pure butter remains clear, and only gives 

 a slight deposit after standing one or one and a half hours. Mar- 

 garine shows a deposit in one or two minutes, and in ten minutes 

 yields a copious precipitate. Mixtures of butter with 10 per 

 cent, of margarine begin to separate in about fifteen minutes. 

 As soon as a few solid particles have fallen, they are withdrawn 

 and examined under the microscope. Genuine butter appears 

 in long, very narrow crystalline rods, often pointed at the ends, 

 sometimes bent, and usually joined together centrally into more 

 or less symmetrical open stars. Margarine crystals consist of 

 bundles of minute needles packed closely toizcther into circk's, 

 sheaves, or dumb-bell-like masses. 



Mercier digests 1 c.c. of melted fat with .30 c.c. of '.)<• per cent, 

 alcohol for five minutes at 50° to 5.") C, the fat and alcohol 

 beiui; well mixed ; and after fifteen to twenty minutes' .standing 

 '20 c.c. of the alcoholic solution are withdrawn, cooled to .30° 

 to 40° C, and filtered ; the fat is then slowly allowed to crystal- 

 lise, and the crystals filtered out and examined l)y the micro- 

 scope. If coco-nut oil is present, little round bunches of crystals 

 consisting of long needles are observed. 



Hinks has also devised a process, which depends on the crystal- 

 lisation of coco-nut oil from alcohol ; 5 c.c. of butter fat are 

 dissolved in 10 c.c. of ether in a test tube, which is then packed 

 with ice. After half an hour, the clear ethereal solution is filtered 

 through a pleated filter, the filtrate evaporated, and the residual 

 fat boiled with three or four times its volume of alcohol (96 to 

 97 per cent, by volume — the strength is important). Complete 

 solution takes place at the boiling point, and the liquid is allowed 

 to cool to room temperature, and then placed in water at 5° C. 

 for fifteen minutes. The alcoholic solution is rapidly filtered 

 into a tube, which is kept at 0° C. for two or three hours. The 

 fiocculent deposit is examined by the microscope, using a magni- 

 fication of 250 to 300 diameters, preferably on a cooled stage ; 

 butter deposits glycerides in round granular masses, but coco- 



