In the foregoing factors concerned in the contamination of milk, it is of course impossible to measure accurately the influence of the different sources of infection, as these are continually subject to variation in every case. As a rule, the most important factors are those pertaining to the utensils and the condition of the animal herself. If these two factors are brought under reasonable control, the major portion of contamination that ordinarily obtains is done away with. The application of the remedial or preventive measures heretofore mentioned will greatly reduce the germ content of the milk. =Cooling of milk on farm.= Bacterial growth is directly related to temperature conditions, and with summer temperatures, such development goes on apace, unless it is checked by early cooling. The larger portion of bacteria that find their way into milk, especially those that are previously in contact with the air, are in a dormant condition, and are therefore not stimulated into immediate growth, unless reasonably high temperatures prevail. In milk, which comes from the animal at blood heat, this growth is greatly stimulated. To counteract this effect, milk should be chilled as soon after milking as possible. If the temperature is immediately lowered to 50° F., or lower, actual cell development is greatly retarded, and the rate of souring, and other fermentative changes thereby diminished. In this country ice is liberally used in accomplishing this result. In Europe, the use of ice is much less common. The employment of such artificial means of refrigeration makes possible the shipment of milk for long distances by rail. New York city now receives milk that is produced in Canada and northeastern Ohio. [Illustration: Fig. 16.--Effect of Cooling Milk.] =Aeration of milk=. The custom has been extensively recommended of subjecting milk to the influence of air in the belief that such exposure permits of the interchange of gases that would improve the quality. In practice, this process, known as aeration, is carried on in different ways. In some cases, air is forced into the milk; in others, the milk is allowed to distribute itself in a thin sheet over a broad surface, falling in drops or tiny streams through the air. Whenever this process is carried on at a temperature lower than that of the milk, it results in more or less rapid cooling. In earlier times, aeration was generally recommended and practiced, especially in connection with the cheese industry, but carefully controlled experiments fail to show that the process exerts any material influence on the rate of germ development. If it is carried out in an atmosphere more or less charged with bacteria, as in the barn or stable, it is more than likely to add to the bacterial content of the milk. While to some extent odors may be eliminated by the process, the custom is not followed so generally now as it used to be some years ago.