300 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
apparatus for making this test. In some the milk is 
permitted to flow through the filter by gravity, while in 
the others it is forced through by air pressure from a 
rubber bulb. According to the quantity of dirt collected 
on the cotton disc the milk is classed as good, medium, 
fair, and bad. The discs are sometimes dried and mailed 
to the producer for his examination. There is no direct 
relation between the quantity of dirt collected on the cot- 
ton disc and the bacterial content. Milk which would be 
classed as good according to this test may have a high 
bacterial content and vice versa. Furthermore, milk pro- 
duced under unclean conditions will test “good ” by this 
test if it is well strained beforehand. 
Another method of testing for dirt is to place the 
milk in a conical glass, or in a vessel with a narrow tube 
at the bottom, and measure the volume of sediment which 
settles to the bottom after a certain period; or the milk 
may be centrifugalized and the sediment measured. 
The visible dirt in milk consists principally of par- 
ticles of feed and litter, manure, hairs, dandruff and dust. 
TEST FOR LACTOSE 
Glage has devised a simple test for lactose which may 
be applied to samples of individual cow’s milk or to 
samples of milk from the different quarters of the udder. 
It is made as follows: Place 8 c.c. of milk and 8 c.c. of 
a 15 per cent. solution of potassium hydroxide in a test 
tube, boil thoroughly and let stand for 10 minutes. When 
the milk contains the normal amount of lactose the mix- 
ture at first becomes pale yellow during the boiling, 
changing quickly to dark yellow, to orange, and then 
to brown; after standing the color of the mixture becomes 
coffee-brown or red-brown. If the quantity of lactose is 
