~Air in barn.~ The atmosphere of the barn where the milking is done may frequently contribute considerable infection. Germ life is incapable of development in the air, but in a dried condition, organisms may retain their vitality for long periods. Anything which contributes to the production of dust in the stable and aids in the stirring up of the same increases the number of organisms to be found in the air (Fig. 11). Thus, the feeding of dry fodder and the bedding of animals with straw adds greatly to the germ life floating in the air. Dust may vary much in its germ content depending upon its origin. Fraser found the dust from corn meal to contain only about one-sixth to one-eighth as much germ life as that from hay or bran.[36] In time most of these dust particles settle to the floor, but where the herd is kept in the barn, the constant movement of the animals keeps these particles more or less in motion. Much can be done by forethought to lessen the germ content of stables. Feeding dry feed should not be done until after milking.[37] In some of the better sanitary dairies, it is customary to have a special milking room that is arranged with special reference to the elimination of all dust. In this way this source of infection may be quite obviated as the air of a clean, still room is relatively free from bacteria, especially if the floor is moistened. It has often been noted that the milk of stall-fed animals does not keep as well as that milked out of doors, a condition in part attributable to the lessened contamination. [Illustration: FIG. 11. Effect of contaminated air. The number of spots indicate the colonies that have developed from the bacteria which fell in 30 seconds on the surface of the gelatin plate (3 inches in diameter). This exposure was made at time the cows were fed.]