92 MILK PRODUCTS 
amination under polarized light with crossed 
nicols (i. e., dark field), when the process butter 
appears mottled, owing to the presence of 
crystals. 
Butter Colors.—Butter and butter-substitutes 
are usually artificially colored. Turmeric and 
annatto or azo-colors allied to methyl-orange are 
used. 
Azo-colors—These may be detected by the 
test devised by Geisler. A small amount of the 
sample, or, better, the fat filtered from it, is mixed 
on a porcelain plate with a little fullers’ earth. 
Azo-colors give promptly a red mass; if they are 
not present, the mixture becomes only yellow or 
light brown. All samples of fullers’ earth are 
not equally active, and tests should be made with 
different samples by using fat known to contain 
the azo-compound until a good specimen of the 
earth is secured. 
For the detection of very minute quantities of 
the color, the sample may be dissolved in light 
petroleum, and the fullers’ earth added to the 
solution, when the pink color will appear as a 
distinct ring or zone at the edge of the deposited 
layer of the reagent. 
Low has proposed the following test for the 
yellow azo-color: A few c.c. of the filtered 
fat are mixed in a large test-tube with an 
equal volume of a mixture of one part strong 
