254 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
In individual milk from diseased cows the refraction 
number has been found to be decreased in tuberculosis, 
febrile diseases, foot and mouth disease, and mastitis. In 
some cases of streptococcic mastitis Obladen found it as 
low as 24.8. The effect of mixing milk from diseased 
cows with normal milk has not been determined, but since 
diseased milk and watered milk are both low in refraction 
there is a possibility of confusion unless a proper investi- 
gation is made. 
DETERMINATION OF THE REACTION 
Litmus Test—A strip of red and a strip of blue lit- 
mus paper are dipped in the milk for a few seconds and 
the color of the wet and dry portions is compared. Nor- 
mal milk, when drawn from the udder, is amphoteric to 
litmus, 7.e., the blue litmus is changed to red and the red 
litmus to blue. If the milk is more acid than normal, the 
blue paper will be colored red but the red paper will not 
be changed, while if the alkalinity is increased the red 
paper will be colored blue and the blue paper will not 
be changed. Individual milk is frequently alkaline to 
litmus in the last stages of lactation; it also gives an 
alkaline reaction in most cases of mastitis, but in some 
cases it is acid. Market milk is usually acid to litmus; it 
is never alkaline except when an alkali has been added. 
The litmus test is only of value as a preliminary test 
for individual milk. 
Mann’s Acidity Test—The apparatus required for 
this test is a 50 c.c. burette with a stopcock, a stand, a 
beaker or a white cup, a stirring rod, and a 50 c.c. pipette. 
The reagents required are a solution of phenolphthalein 
and a one-tenth normal solution of sodium hydroxide. 
The phenolphthalein solution is prepared by dissolving 
