276 THE PHYSICAL EXAMINATION OF BUTTER FAT. 



Molecular Specific Volumes. A method of calculating 

 which will sometimes be of use is to deduce the specific volume 

 by dividing the density into 1, and to multiply the figure thus 

 obtained by the potash absorption and to divide the result by 

 19-5. 



The mean figure thus obtained for butter is 1-2766, .and for 

 margarine 1-1641 at 37-8 C. If the butter is adulterated with 

 beef or other animal fat, the percentage of adulteration calcu- 

 lated with this figure will agree fairly well with that calculated 

 from other determinations. If vegetable oils have been used, 

 the percentage deduced thus is considerably more. 



Refractive Index. 



The Oleo-refractometer. When light passes from one 

 medium to another it passes only in a straight line when it falls 

 perpendicular to the surface separating the two media. If it 

 passes through at an angle to the surface, it is bent or refracted, 

 and the ratio of the sine of the angle made by the path of the 

 ray with the perpendicular to the surface in the first medium 

 to the sine of the angle made by the path in the second 

 medium with the perpendicular is a constant, known as the 

 index of refraction. As the sine of an angle of 90 is 1, it is seen 

 that the ratio between the sine of the angle at which light is first 

 reflected and 1 is the index of refraction ; this angle is termed 

 the angle of total reflection. As it is more convenient to measure 

 this than to measure the two angles, and deduce the ratio of the 

 sines, in practice the angle of total reflection is frequently 

 measured. 



Miiller was the first to apply the determination of the refrac- 

 tive index to the analysis of butter. He allowed the butter to 

 solidify slowly, absorbed the liquid portion with filter paper, 

 extracted this with ether, and examined it in Abbe's refracto- 

 meter, an instrument which measures the angle of total reflection. 

 Skalweit examined this method and showed that it was important 

 to operate at a fixed temperature. 



Owing to the difficulty of maintaining a fixed temperature in 

 Abbe's refractometer, this method was not much used till special 

 instruments were devised. 



Amagat and Jean have devised an oleo-refractometer for deter- 

 mining the refractive index of oils and fats (Fig. 37) ; it consists 

 of a collimator, a hollow prism with sides inclined at an angle of 

 107, and a telescope furnished with an arbitrary glass scale 

 placed in the focus of the eye-piece. In the collimator is placed 

 a piece of opaque substance, which cuts off the light from one- 

 half of the field. If the prism and the space outside between 

 the collimator and the telescope be filled with the same liquid, 



