42 MILK 



other solids increase it, it is possible to take off 

 a small amount of the former and add some 

 water without disturbing the sp. gr., but, of 

 course, the above analytical methods wiU detect 

 this procedure. It is now admitted that, except 

 in cases of wide departure from the usual limits, 

 the adulteration of milk cannot be detected by 

 the sp. gr. alone but the employment of a care- 

 fully graduated lactometer is of use in routine 

 milk inspection. 



Direct Detection of Added Water. Serum-refrac- 

 tion. — Of late years several methods have been 

 proposed for this purpose but most of them have 

 no positive value and have not come into general 

 use. The refractive index of the whey (milk- 

 serum) offers a rapid and satifactory method for 

 detecting watering. Several methods of pre- 

 paring this whey have been proposed, but 

 Lythgoe has found, as the result of extended 

 experience, the following to be satisfactory. 



Dissolve 7.25 grams of crystallized copper sulfate 

 in water and dilute to 1000 c.c. If this solution 

 does not refract 36 on the scale of the immersion 

 refractometer at 20°, add water or copper sulfate 

 until the desired result is obtained. To 8 c.c. 

 of the copper solution add 32 c.c. of milk. Shake 

 well and pour upon a dry filter. When the filtrate 

 begins to come through clear, change the receiver, 

 pour the small quantity of cloudy filtrate upon 



