METHODS OF EXAMINING MILK 249 
contain up to 1 to 2 mg. per litre from this source. Rievel 
also states that a positive reaction may occur with milk 
soiled with manure, but Tillmans has never found dirty 
milk to give any indication of containing more nitrates 
or nitrites than milk from vessels that had been washed 
with water rich in nitrates or nitrites. A negative reac- 
tion to the nitrate test is no evidence that the milk has not 
been watered, as water free from these substances may 
be used for dilution. According to Utz, milk to be 
tested for nitrates should be boiled unless the test can 
be made at once, because the action of the lactic acid- 
forming bacteria may reduce the nitrates or cause them 
to disappear entirely. 
DETECTION OF THE USUAL ADULTERATIONS 
When milk is adulterated, it is usually by partial 
skimming, dilution with skimmed milk, or dilution with 
water. The changes caused by these various forms of 
adulteration are shown in the following table, which is 
taken from Jensen’s Milk Hygiene: 
. Specific 
Bpeats Per- Percent- gravity Presence 
Breve | centage! Seid, of the | < the | centage! or absence 
of the - 
: of fat milk of ash | of nitrate 
milk not fat solids total 
1.029 to 
Normal milk......... 1.034 | 3 to 5 | 8.5 to | 1.80 to | 20 to | 0.7 to 0 
aver. 10.5 1.34 34 0.75 
1.032 
Skimmed or diluted Very : 
with skimmed milk‘| Higher | Lower] slightly | Higher | Lower|Higher| 0 
higher 
Water added.......-. { Lower | Lower| Lower dine ie Lower|++ or 0, 
Skimmed and water f| Little 
adied  Blgipiaia soe HERES { change | Lower| Lower | Higher | Lower| Lower + or 0 
