There are three possible ways by which milk may be preserved: (1) The removal of bacteria that have gained entrance to it; (2) The prevention of growth of the contained bacteria; (3) The destruction of the contained organisms. In practice at least two and sometimes all of these methods are employed. The prevention of contamination, a subject discussed in Chapter III is in reality one of the most efficient means of preserving milk. In milk production, as elsewhere, prevention is preferable to cure. Milk produced under such conditions that its germ content is but a few thousand per cubic centimeter will keep much longer than that handled in the ordinary manner. It might naturally be supposed that any method by which dirt is removed from milk would improve the keeping quality of milk, due to the reduction of bacteria, yet while the straining of the milk at the time of milking removes dirt of various kinds, it does not appreciably enhance the keeping quality, owing to the fact that the bacteria adherent to the dirt particles are washed off in straining, and pass through the pores of the strainer. =Filtration of milk.= It is possible to remove all bacteria from water and other fluids and thus render them sterile by passing through filters of unglazed porcelain. This process can not be used with milk for the fat globules are larger than the bacteria (see Fig. 6) and any process that would remove the latter would also remove the former. The term "filtration" is applied to a process used in some European cities for the removal of the insoluble dirt that has been introduced into the milk. Suitable containers are filled with layers of coarse sand at the bottom and with finer sand at the top. The milk is introduced at the bottom and is forced upward through the sand. Such a filtering process is a very efficient means of removing the dirt; but unless the filters are kept scrupulously clean, the bacteria are likely to grow in the filtering material, so that the number of organisms in the milk may actually be increased by the filtering process. It is necessary to remove the sand daily and thoroughly wash and sterilize the same. The extra care required in keeping these sand filters in sanitary condition has been the great objection to their employment in this country. Filters of other material such as cellulose have been employed but with no marked success.