278 THE PHYSICAL EXAMINATION OF BUTTER FAT. 



Jean gives the following method for testing butter : Melt 

 from 25 to 30 grammes of the butter in a porcelain dish at a 

 temperature not exceeding 50 C. ; stir well with a pinch or two 

 of gypsum, and allow to settle out at about the same tempera- 

 ture. Then decant the supernatant fat through a hot water 

 funnel plugged with cotton wool, and pour (while warm) into 

 the prism. Observe the deviation at 45. Genuine butter gives 

 a deviation of about 30 to the left, while margarine gives about 

 15, and coconut oil about 59. Lobry de Bruyn has shown 

 that genuine butters may show a deviation of 25 to the left. 



It is evident that the addition of a mixture of coconut oil 

 and margarine would give a figure equal to that of butter. Muter 

 has, however, shown that the figure given in the oleo-refracto- 

 meter has a relation to the Reichert figure, which would be much 

 disturbed by such a mixture. 



Muter's relation is expressed by Table L. 



TABLE L. 



A deviation of 36 is accompanied by a Reichert figure of 16-0. 

 -35 15-25. 



-34 

 -33 



-31 

 -30 

 -29 



14-5. 



13-75. 



13-0. 



12-25. 



11-5. 



10-75. 



Butyro-refractometer. Zeiss' butyro-refractometer (Fig. 38) 

 measures the angle of total reflection and is a modification of the 

 well-known Abbe refractometer. It consists of two prisms of 

 glass, hinged so that they can be separated. The light enters 

 at the bottom, passes through the prisms, and is viewed through 

 a telescope having a fixed scale in the focus of the eye-piece. 

 The prisms are provided with a jacket, through which water, the 

 temperature of which is indicated by a thermometer, is passed. 

 A drop of the filtered fat is placed on the glass surface of the 

 lower prism, spread evenly over it, and the prism closed ; the 

 reflector is so adjusted as to reflect clear daylight or lamplight 

 through the prisms, and the refractive index in scale degrees is 

 read off. 



This instrument is extremely rapid, as a determination, in- 

 cluding reading of the temperature and scale degrees, does not 

 take more than a minute. After use, the instrument should be 

 cleaned by rubbing off the fat with a duster, and polishing 

 the prisms with a clean linen cloth slightly moistened with 

 alcohol. 



