_Nutritive Medium._ In the very earliest days of the study of micro-organisms it was observed that they mostly congregate where there is pabulum for their nourishment. The reason why fluids such as milk, and dead animal matter such as a carcass, and living tissues such as a man's body contain so many microbes is because each of these three media is favourable to their growth. Milk affords almost an ideal food and environment for microbes. Its temperature and constitution frequently meet their requirements. Dead animal matter, too, yields a rich diet for some species (saprophytes). In the living tissues bacteria obtain not only nutriment, but a favourable temperature and moisture. Outside the human body it has been the endeavour of bacteriologists to provide media as like the above as possible, and containing many of the same elements of food. Thus the life-history may be carried on outside the body and under observation. By means of cover-glass preparations for the microscope we are able to study the form, size, motility, flagella, spore formation, and peculiarities of staining, all of which characters aid us in determining to what species the organism under examination belongs. By means of artificial nutrient media we may further learn the characters of the organism in "pure culture,"[7] its favourable temperature, its power or otherwise of liquefaction, the curdling milk, or of gas production, its behaviour towards oxygen, its power of producing indol, pigment, and chemical bodies, as well as its thermal death point and resistance to light and disinfectants. It is well known that under artificial cultivation an organism may be greatly modified in its morphology and physiology, and yet its conformity to type remains much more marked than any degeneration which may occur.