§ 136. MILK. 265 



VogePs optical milk test. — This i^rocess meets with 

 very general acceptance. It depends upon the fact that 

 the light is intercepted by water containing a smaller 

 proportion of milk, the richer the milk is in butter. 



The apparatus required consists of a measuring flask, 

 with a mark on the neck, indicating a capacity of 100 c.c, 

 a test-glass, for holding a sample of the milk and w^ater 

 between the eye and the light, which should have parallel 

 glass sides, ^\ ^ cm. apart, so that the thickness of the layer 

 of milk looked through will be exactly ^ 1^ cm., a pipette 

 graduated in ^|^ cubic centimetres, and holding 4-5 c.c, 

 and a box about 16 cm. long and wide, with a slit in one 

 side, in front of which, and 40 cm. distant, the stearin 

 candle is placed ; the opposite side of this box is so cut 

 out to fit the face that, when the glass containing the 

 milk is put in the box, all light can be excluded while an 

 observation is made, except that coming through the slit 

 from the candle ; the inside of the box should be painted 

 black. 



To perform the test, fill the 100 c.c. flask with distilled 

 water up to the mark, add to it 3 c.c. of the well-stirred 

 sample of the cooled milk, and mix the two together 

 thoroughly by vigorous agitation ; fill the test-glass with 

 this mixture, put it in the dark box, and make the obser- 

 vation, placing the eye close to the test-glass, and the 

 candle against a dark background. If the light can be 

 seen, pour the test sample back into the flask, add ^ 1^ c.c. 

 more of milk, and make another observation ; continueto 

 operate in this manner, adding ^ L oi* Mi ^•^* ^^ ™^^^ ^^^^ 

 time, until the light is no longer visible. 



The relation between the number of cubic centimetres 

 of milk required and the per cent of butter is given in 

 Table IX. This per cent is calculated by the formula 

 23:? 4- 0.23, in which 2/ = the number of cubic centimetres 

 of milk required. If 5.5 c.c. or more of milk were used 

 to produce opacity, it is probable that the milk was 



