In carrying out the microscopical part of the work, not only is the organism examined in a living condition, but colored preparations are made by using solutions of anilin dyes as staining agents. These are of great service in bringing out almost imperceptible differences. The art of staining has been carried to the highest degree of perfection in bacteriology, especially in the detection of germs that are found in diseased tissues in the animal or human body. In studying the peculiarities of any special organism, not only is it necessary that these cultural and microscopical characters should be closely observed, but special experiments must be made in different ways, in order to determine any special properties that the germ may possess. Thus, the ability of any form to act as a fermentative organism can be tested by fermentation experiments; the property of causing disease, studied by the inoculation of pure cultures into experimental animals, like rabbits, guinea pigs and white mice. The methods of the bacteriologist in his laboratory are in their effect not dissimilar to those which the farmer employs in securing his crop of pure-bred grain. The laboratory farmer kills the weed seeds in his culture field by the application of heat. His field, which is embraced in his culture dish, has been fertilized and prepared by the addition of certain favorable ingredients. When he has garnered his crop, he maintains its purity by keeping his selected seed, the pure culture, free from all contamination. The dairyman, even though he may not expect to carry on the detailed operations of the laboratory, will understand the reason for the directions which he is often required to follow much better if he knows how the simple operations of the laboratory are carried out. For a fuller knowledge of these matters, the reader is referred to the special texts on bacteriology. CHAPTER III. CONTAMINATION OF MILK. =Spoiling of milk.= Materials of animal origin are peculiarly prone to undergo changes, rendering them unfit for use, and of these, milk is exceedingly susceptible to such changes. This is due to the fact that the composition of milk is especially adapted to bacterial growth, and that the opportunity for entrance of such organisms is likewise such as to permit of abundant contamination. The consequence is that milk readily undergoes fermentative changes, due to the development of one or another type of micro-organism. =Milk, a suitable bacterial food.= While milk is designed by nature for the nourishment of mammalian life, it is, curiously enough, equally well adapted to the growth of these lowest forms of vegetable life. The nutritive substances required by bacteria are here sufficiently dilute to make possible rapid growth.