All chlorophyll-free organisms act in a transforming and disturbing manner on the organic compounds from which they obtain their nourishment, and while they themselves grow and multiply, they produce, each after its kind, compounds of a less degree of complexity, _i.e._ they produce _fermentation_, _putrefaction_, sometimes the formation of _poisons_, and in living beings often _disease_. Those organisms which produce fermentation are called _ferments_; this word, however, is also employed for similar transformations in purely chemical materials (inorganic ferments or enzymes). Many organic (“living”) ferments, among which are Yeast-cells and Bacteria, give off during their development certain inorganic and soluble ferments (enzymes) which may produce other transformations without themselves being changed. Different organisms may produce in the same substratum different kinds of transformation; alcoholic fermentation may for instance be produced by different species of Fungi, but in different proportions, and the same species produces in different substrata, different transformations (_e.g._ the Vinegar-bacteria oxydize diluted alcohol to vinegar, and eventually to carbonic acid and water). In the study of Bacteria it is absolutely necessary to sterilize the vessels employed in cultivation, the apparatus, and nutrient solutions, _i.e._ to free them from Bacteria germs and also to preserve the cultures from the intrusion of any foreign germs (“pure-cultures”). A firm, transparent, nutritive medium is frequently employed. This may be prepared by adding to the nutrient solutions (broth) either gelatine, or--when the Bacteria are to be cultivated at blood-heat--serum of sheep’s or calf’s blood, agar-agar or carragen; serum alone may in itself serve as a nutrient medium. The so-called “plate-cultures” are frequently employed, _i.e._ the germs are isolated by shaking them with the melted liquid nutrient gelatine, which is then spread on a glass plate and allowed to coagulate; when later on the individual germs grow into colonies, these remain separate in the solid substratum and it is easy to pursue their further development. Similar plate-cultures may also be cultivated in test-tubes and on microscopic slides. The slides and glass plates must be placed in “moist chambers” free from Bacteria. By sowing a few cells (if possible one) using a fine platinum wire, pure cultures for further investigation may be obtained. In order to prove the relationship between pathogenic Bacteria and certain diseases, the experimental production of pathogenic Bacteria by the inoculation of Bacteria from pure cultures into healthy animals, is very important.