270 THE PHYSICAL EXAMINATION OF BUTTER FAT. 



It is apparent that this test must be used with reservation, 

 but it is without doubt of use as corroborative evidence in cases 

 where other analytical data are not absolutely conclusive. 



The method is carried out as follows : The outer portions of 

 a piece of butter are removed, and a piece about the size of a 

 pin's head is transferred from the freshly exposed surface to a 

 clean microscope slide. A cover glass is placed on the top, and 

 the butter spread out by gentle pressure on the upper surface of 

 the cover. The slide is placed on the stage of a microscope fitted 

 with crossed Nicol prisms, and examined with a 1-inch objective 

 or higher power. To exclude light from the upper surface a 

 blackened cardboard tube may be placed over the slide in such a 

 manner that the objective dips into it, and the light falling on 

 the upper portion of the slide is cut off. When pure butter is 



Fig. 35, Butter under Polarised Fig. 36. Margarine under Polarised 



Light. Light. 



examined the field is uniformly dark, and only with the greatest 

 difficulty can any structure be distinguished. When margarine 

 is present certain portions of the field have a bright appearance, 

 and indistinct crystalline forms can be made out. If any distinct 

 and bright crystals are seen, the Nicol prisms should be turned 

 parallel, and the slide examined in that spot in order to see 

 whether salt or other crystalline matter is present ; there is not 

 much difficulty in distinguishing this owing to its great refrac- 

 tive power. The slide should be moved about to examine all 

 parts of it, as, in cases of small amounts of adulteration, the 

 margarine is not distributed equally throughout, and two or 

 more portions from different parts of the sample may be examined. 

 As a check, a selenite plate (a crystalline form of calcium 

 sulphate, which possesses the property of rotatory dispersion to 



