1. _Dirt or sediment test._ This is made by filtering a pint of the mixed milk through a small disc of absorbent cotton. The insoluble dirt is retained and imparts a color to the cotton, the shade of which is dependent on the amount of dirt (P. 45). Since it is impossible to have dirt without bacteria, it is evident that milks containing a large amount of dirt will be high in bacteria. The reverse, however, is not necessarily true. [Illustration: Fig. 40.--Good Milk. A plate culture inoculated with 1/100 of a cubic centimeter of milk containing 67 colonies, which equals 6,700 bacteria per cubic centimeter of milk. Such milk will keep well.] 2. _Acidity test._ The acidity of the milk is also an indication of its bacterial content. If the acidity has increased, above the normal for fresh milk, the bacterial content is certain to be high, and the keeping quality poor. An acidity above 0.2 per cent in market milk is to be avoided, as an increase in acidity is always preceded by a great increase of bacteria. Whether the acidity is above or below this point can be rapidly and easily determined at the receiving station by a modification of the Farrington acid test. Dissolve one alkaline tablet in an ounce of water. A unit volume of this solution added to a unit volume of milk is equal to 0.1 per cent of acidity. If two measures are provided,--one for the alkaline solution holding just twice as much as that used for the milk, the approximate acidity can be quickly determined by mixing a measure of each in a common white cup. If the acidity is above 0.2 per cent the color will remain white; if a pink color develops, it indicates an acidity less than this amount. This test is also useful in the selection of milk or cream that is to be used for special purposes, such as pasteurization. [Illustration: Fig. 41.--Poor Milk. A plate culture inoculated with 1/1000 of a cubic centimeter of market milk containing 1,680 colonies, which equals 1,680,000 bacteria per cubic centimeter. Such milk has poor keeping qualities.] 3. _Alcohol test._ A test giving similar information is made by adding two parts of 70 per cent alcohol to one part of milk, and noting whether curdling occurs. 4. _Curd test._ The curd test described on p. 100 gives no indication of the number of bacteria present, only concerning the types present. It has been proposed to combine the fermentation test with the reduction test referred to below and thus gain some idea of, not only the number, but the kinds of bacteria present.