REFRACTION. 281 



This chart is easy to make, and still easier to use, and the 

 author has found it to give very accurate results over a con- 

 siderable range of temperature, not only for butter, but also 

 for other oils and fats, and for the standard fluid. 



The author has found that genuine butters vary from 43-7 

 (in a sample giving a Reichert value of 16-0 c.c.), to 49-0 (in a 

 sample giving a Reichert value of 10-5 c.c.), and average 46-0 

 at a temperature of 35 C. The value 47-0 has been proposed 

 as a practical limit. 



The equivalent at other temperatures of this limit is as 

 follows : 



Temperature. 

 25 

 30 

 35 



Scale Division. 

 52-5 

 49-8 

 47-0 



Temperature. 

 40 

 45 



Scale Division. 

 44-2 

 41-5 



Some importance has been attached to the colour observed at 

 the edge of the dividing line, and a blue colour has been alleged 

 to be indicative of margarine. In the author's experience this 

 property is valueless. Thus the sample giving a reading of 43*7 

 was tinged red, and that giving 49-0 was tinged blue, though 

 both were authenticated as genuine butters. 



Margarine has a value of about 52 at 35, coconut oil of 

 41, and cotton-seed oil of 61. 



The remarks made upon the oleo-refractometer apply equally 

 well to the butyro-refractometer, except that the actual values 

 are not identical. 



The author's experience confirms that of Mansfeld, and shows 

 that, while Muter's relation is, broadly speaking, correct, there 

 are differences so large between the refractive index found and 

 that calculated on the assumption that this property follows the 

 Reichert value, that the rule cannot be depended upon. The 

 refractive index is a property which is much more nearly related 

 to the iodine absorption, or, in other words, to the unsaturated 

 carbon atoms. 



Though a very convenient test, it has but little value alone, 

 unless the value is below the average, 46 at 35 C., but in the 

 presence of coconut oil and margarine an adulterated butter 

 may give a normal figure. When a butter is adulterated with 

 vegetable oils e.g., cotton seed its indications are of some 

 value. It is also useful in detecting coconut oil, but its value 

 is chiefly corroborative. 



A standard fluid (normal flussigkrti) is supplied with the instru- 

 ment, and the readings of the scale should be checked from 

 time to time by its use. The point at which the dividing line 

 should lie at 35 C. is marked in the instrument, and the scale 



