METHODS OF EXAMINING MILK 299 
(176° F.), a blue ring is formed at the point of contact 
of the two fluids. If the milk has been heated to 80° C. 
(176° F.) or above, no color change occurs. The guaiac- 
wood tincture is more reliable than the guaiac-resin tinc- 
ture of the U. S. Pharmacopceia. The addition of a few 
drops of a weak aqueous solution of hydrogen peroxide 
after the tincture of guaiac has been run into the test 
tube increases the reliability and sharpness of the test 
(Zinc), although if the guaiac tincture is “ripe,” a 
change which takes place with age, the hydrogen peroxide 
is not necessary. When the test is made with the tinc- 
ture alone, the activity of the latter should be tested 
against known raw milk. If the hydrogen peroxide is 
added to the milk before the guaiac tincture, or if too 
much is added, the reaction will be retarded or suppressed. 
When milk is strongly acid it will not give the color 
reaction at all, or the reaction will be faint or delayed. 
Lime water should be added to such milk and to butter- 
milk before testing. Cold milk may not react at all 
or very slowly. An excess of hydrogen peroxide will pre- 
vent the reaction. Heated milk to which formalin has 
been added will give a color reaction to the Storch test 
like raw milk, but formalin has no influence on the guaiac 
test when it is present in the quantity used to preserve 
milk. The guaiac test is also not affected by sodium 
bicarbonate, borax, boric acid, and salicylic acid, but the 
presence of potassium bichromate causes heated milk to 
react like raw milk and intensifies the color reaction of 
raw milk. 
EXAMINATION FOR DIRT 
The milk is filtered through small cotton discs and 
the larger particles of dirt present are collected on the 
surface of the disc. There are several different forms of 
