~Necessity of bacterial masses for study.~ The bacteria are so extremely small that it is impossible to study individual germs separately without the aid of first-class microscopes. For this reason, but little advance was made in the knowledge of these lower forms of plant life, until the introduction of culture methods, whereby a single organism could be cultivated and the progeny of this cell increased to such an extent in a short course of time, that they would be visible to the unaided eye. This is done by growing the bacteria in masses on various kinds of food media that are prepared for the purpose, but inasmuch as bacteria are so universally distributed, it becomes an impossibility to cultivate any special form, unless the medium in which they are grown is first freed from all pre-existing forms of germ life. To accomplish this, it is necessary to subject the nutrient medium to some method of sterilization, such as heat or filtration, whereby all life is completely destroyed or eliminated. Such material after it has been rendered germ-free is kept in sterilized glass tubes and flasks, and is protected from infection by cotton stoppers. ~Culture media.~ For culture media, many different substances are employed. In fact, bacteria will grow on almost any organic substance whether it is solid or fluid, provided the other essential conditions of growth are furnished. The food substances that are used for culture purposes are divided into two classes; solids and liquids. Solid media may be either permanently solid like potatoes, or they may retain their solid properties only at certain temperatures like gelatin or agar. The latter two are of utmost importance in bacteriological research, for their use, which was introduced by Koch, permits the separation of the different forms that may happen to be in any mixture. Gelatin is used advantageously because the majority of bacteria present wider differences due to growth upon this medium than upon any other. It remains solid at ordinary temperatures, becoming liquid at about 70 deg. F. Agar, a gelatinous product derived from a Japanese sea-weed, has a much higher melting point, and can be successfully used, especially with those organisms whose optimum growth point is above the melting point of gelatin. Besides these solid media, different liquid substances are extensively used, such as beef broth, milk, and infusions of various vegetable and animal tissues. Skim-milk is of especial value in studying the milk bacteria and may be used in its natural condition, or a few drops of litmus solution may be added in order to detect any change in its chemical reaction due to the bacteria.