To further study the peculiarities of different germs, the separate colonies are transferred to other sterile tubes of culture material and thus _pure cultures_ of the various germs are secured. These cultures then serve as a basis for continued study and must be planted and grown upon all the different kinds of media that are obtainable. In this way the slight variations in the growth of different forms are detected and the peculiar characteristics are determined, so that the student is able to recognize this form when he meets it again. These culture methods are of essential importance in bacteriology, as it is the only way in which it is possible to secure a quantity of germs of the same kind. ~The microscope in bacterial investigation.~ In order to verify the purity of the cultures, the microscope is in constant demand throughout all the different stages of the isolating process. For this purpose, it is essential that the instrument used shall be one of strong magnifying powers (600-800 diameters), combined with sharp definition. [Illustration: FIG. 4. Pure cultures of different kinds of bacteria in gelatin tubes. _a_, growth slight in this medium; _b_, growth copious at and near surface. Fine parallel filaments growing out into medium liquefying at surface; _c_, a rapid liquefying form; _d_, a gas-producing form that grows equally well in lower part of tube as at surface (facultative anaerobe); _e_, an obligate anaerobe, that develops only in absence of air.] The microscopical examination of any germ is quite as essential as the determination of culture characteristics; in fact, the two must go hand in hand. The examination reveals not only the form and size of the individual germs, but the manner in which they are united with each other, as well as any peculiarities of movement that they may possess. In carrying out the microscopical part of the work, not only is the organism examined in a living condition, but 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.