=Factory contamination.= As the time element is of importance in the production of troubles due to bacteria, it follows that infection of milk on the farm is fraught with more consequence than factory contamination, as the organisms introduced would have a longer period of development. Nevertheless, the conditions in the factory are by no means to be ignored, as they not infrequently permit the milk to become seeded with highly undesirable types. A much more rigid control can be exercised in the factory, where steam is at hand as an aid in the destruction of organisms. In the cleaning of pumps and pipes, steam is absolutely necessary to keep such apparatus in a sanitary condition. The water supply of the factory is a matter of prime importance, as water is used so extensively in all factory operations. When taken from a shallow well, especially if surface drainage from the factory is possible, the water may be contaminated to such an extent as to introduce undesirable bacteria in such numbers that the normal course of fermentation may be changed. The quality of the water, aside from flavor, can best be determined by making a curd test (p. 99) which is done by adding some of the water to boiled milk, and incubating the same. If "gassy" fermentations occur, it signifies an abnormal condition. In deep wells, pumped as thoroughly as is generally the case with factory wells, the germ content should be very low, ranging from a few score to a few hundred bacteria per cubic centimeter at most. The danger from ice is much less, for the reason that good daily practice does not sanction using ice directly in contact with milk or cream. Then, too, water is largely purified in the process of freezing, although if secured from a polluted source, reliance should not be placed in this method of purification, for even freezing does not destroy all vegetating bacteria. The ordinary house fly is an important source of contamination in creameries, cheese factories and city milk plants. They are of importance not only in increasing the number of fermentative bacteria in milk but they may serve to contaminate it with disease-producing organisms. The windows of all places where milk is handled, whether on the farm or elsewhere should be screened. It should be kept in mind in the handling of milk and other dairy products that human food is being prepared and that cleanliness is desirable from every point of view, and that the methods of handling and production should compare with those used in the preparation of foods which like milk cannot be cleaned when once polluted. Desirability, keeping quality, healthfulness and the value of every product made from milk depends upon the extent and amount of contamination. CHAPTER IV. INFECTION OF MILK WITH PATHOGENIC BACTERIA.