398 
Fat . — The quantity of butter fat in the milk was determined by 
the Babcock centrifugal method. This is the most rapid method > 
known for the determination of fat in milk. It compares very 
favorably as to accuracy with the most exact methods now known for 
the determination of fat in milk, and it is the method ordinarily 
employed in practice for this purpose. 
Lactose . — The amount of lactose in the several samples of milk was 1 
determined polarimetrically after the removal of the milk proteids ! 
by means of an acid solution of mercuric nitrate. ; 
Acidity . — The acidity of the milk was determined by titrating 50 j 
cubic centimeters of the samj^le with tenth-normal sodium hydroxide, i 
using phenolphthalein as the indicator. j 
One cubic centimeter of tenth-normal sodium hydroxide, contain- < 
ing 0.001 gram of sodium hydroxide, is equivalent to 0.009 gram of ] 
lactic acid. Hence each cubic centimeter of tenth-normal sodium 
hydroxide required by the 50-cubic-centimeter sample of milk is | 
equivalent to 0.018 per cent of lactic acid. In order, therefore, to J 
obtain the per cent of acidity of the sample we multiply the number | 
of cubic centimeters of tenth-normal .sodium hydroxide required for i 
neutralization by 0.018. The product is the acidity of the milk in ' 
percentage of lactic acid. i 
Thoerner (2) has suggested as a practical limit for wholesome milk | 
an acidity equal to one-fifth of the volume of the milk in cubic cen- i 
timeters of tenth-normal caustic soda. This would correspond to an j 
acidity of 0.18 per cent of lactic acid. According to Van Slyke (3), i 
the average acidity of English market milk, supposed to be 12 to 18 
hours old, is 0.18 per cent, and of German milk 0.13 to 0.18 per cent. 
According to this author, market milk should not in any case contain 
over 0.2 per cent total acidity when it reaches the consumer, and 
generally should be under 0.15 per cent. According to Tuley (4), 
the milk of swill- fed cows is hyperacid. 
Di?d. — The quantity of dirt or suspended matter in the milk may 
be estimated either gravimetrically (Kenk, quoted by Ott (5)) or 
volumetrically (Van Slyke). The gravimetric method requires a 
large volume of milk and also requires considerable time. The Amlu- 
metric method is rapid, and only small amounts of milk are required. 
For this reason the latter method was employed. Fifteen cubic centi- 
meters of the sample was placed in a Bausch and Lomb graduated 
centrifuge tube. The samples were then centrifugalized for five 
minutes. The dirt then collects on the bottom of the tube, and the 
volume of it is read. From these readings the per cent of dirt is 
calculated. 
