METOHDS OF EXAMINING MILK 253 



Rothenfusser it reached 41.5. The addition of a small 

 quantity of water to milk lowers the refraction number, 

 but on account of the wide range in the refraction number 

 of normal market milk it is necessary, in order to detect 

 slight additions of water, to have the herd under suspicion 

 milked under supervision and to take a sample of the 

 milk and compare the refraction number of this sample 

 with the refraction number of the sample of market milk. 

 The daily variation in the refraction number of the milk 

 of a herd will not exceed 0.1 to 0.55. Changes of feed 

 have only a slight influence and incomplete milking has 

 no effect. When the refraction number is as low as 36.5 

 or below the milk may be unhesitatingly pronounced 

 watered. 



The refraction number is not affected by the per 

 cent, of fat. A sample of milk taken from the top of a 

 can and one taken from the bottom of the same can will 

 both have exactly the same refraction number, although 

 there will be a great difference in the fat per cent. This is 

 an important advantage, because when a charge that milk 

 has been watered is based on the specific gravity and fat 

 per cent, the claim is often made that the sample was 

 taken from the bottom of the can or after the top milk 

 had been sold. 



The presence of boric acid and borax in the propor- 

 tion usually used to preserve milk (0.1 to 0.2 per cent.) 

 will increase the refraction of the milk serum. Salicylic 

 acid also increases the refraction when present in as 

 small amount as 0.05 per cent, which is the quantity used 

 as a preservative, but formaldehyde causes no increase 

 when present in the amount ordinarily used as a preserva- 

 tive. Sodium bicarbonate does not have any effect when 

 added up to 0.1 per cent. 



