This is explained as either: (1) a period of selection within which types of bacteria entering by accident and unadapted for growth die off; or (2) an actual weak antiseptic power in the milk-serum itself; or (3) the forming of clusters by the bacteria and so reducing the count. In working on a small scale or on an experimental basis, this property at times introduces a factor of difficulty or error which is not to be lost sight of in the selection of the milk for such purposes. +35. Sources and control of bacteria in milk.+--Most of the bacterial infection of milk is due to lack of care in handling. Some of the common sources[15] of contamination are: the air in the stable; the cow's body; the milker; the utensils; the method of handling the milk after it is drawn from the cow; unclean cheese factory conditions. Since bacteria cause various kinds of fermentation, not only in the milk but in the products manufactured from it, the question of their control is of prime importance. There are two ways in which the bacterial growth in milk used for cheese-making may be controlled: (1) prevention of infection; (2) the retardation of their development when present. The former is accomplished by strict cleanliness, the latter by adequate cooling. +36. The cow.+--The body of the cow may be a source of bacterial contamination. Bacteria adhere to the hair of the animal, and to the scales of the skin, and during the process of milking these are very liable to fall into the milk. To prevent this, the cow should be curried to remove all loose material and hair. Just before milking, the udder and flank should be wiped with a damp cloth; this removes some of the material, and causes the remainder to adhere to the cow. +37. Stable air.+--If the air of the stable is not clean, it will be a source of contamination. Particles of dust floating in the air carry more or less bacteria, and these fall into the milk during the process of milking. To keep the stable air free from dust at milking time, all operations which stir up dust, such as feeding, brushing the cows, cleaning the floor, should be practiced after milking or long enough before so that the dust will have settled. It is a good plan to close the doors and to sprinkle the floor just before milking. +38. The milker+ himself may be a source of contamination. He should be clean and wear clean clothing. The hands should not be wet with milk during milking. [Illustration: FIG. 3.--Types of small-top milk pails.]