Food of a definite reaction is needed for the growth of bacteria. As a general rule growth is most active in media which react slightly acid to phenolphthalein--that is, neutral or faintly alkaline to litmus. Mould growth, on the other hand, is most vigourous in media that are strongly acid to phenolphthalein. ~Environment.~--The influence of physical agents upon bacterial life and growth is strongly marked. 1. _Atmosphere._--The presence of _oxygen_ is necessary for the growth of some bacteria, and death follows when the supply is cut off. Such organisms are termed _obligate aerobes_. Some bacteria appear to thrive equally well whether supplied with or deprived of oxygen. These are termed _facultative anaerobes_. A third class will only live and multiply when the access of free oxygen is completely excluded. These are termed _obligate anaerobes_. 2. _Temperature._--Practically no bacterial growth occurs below 5°C, and very little above 40° C. 30°C. to 37° C is the most favorable for the large majority of micro-organisms. The maximum and minimum temperatures at which growth takes place, as well as the optimum, are fairly constant for each bacterium. Bacteria have been classified, according to their optimum temperature, into-- MIN. OPT. MAX. 1. Psychrophilic bacteria (chiefly water organisms) 0° C. 15° C. 30°C. 2. Mesophilic bacteria (includes pathogenic bacteria) 15° C. 37° C. 45°C. 3. Thermophilic bacteria 45° C. 55° C. 70°C. The thermal death-point of an organism is another biological constant; and is that temperature which causes the death of the vegetative forms when the exposure is continued for a period of ten minutes (see pages 298-301). 3. _Light._--Many organisms are indifferent to the presence of light. On the other hand, light frequently impedes growth, and alters to a greater or lesser extent the biochemical characters of the organisms--e. g., chromogenicity or power of liquefaction. Pathogenic bacteria undergo a progressive loss of virulence when cultivated in the presence of light. 4. _Movements._--Movements, if slight and simply of a flowing character, do not appear to injuriously affect the growth of bacteria; but violent agitation, such as shaking, absolutely kills them. A condition of perfect rest would seem to be that most conducive to bacterial growth. ~The Metabolic Products of Bacteria.~--_Pigment Production._--Many micro-organisms produce one or more vivid pigments--yellow, orange, red, violet, fluorescent, etc.--during the course of their life and growth. The colouring matter usually exists as an intercellular excrementitious substance. Occasionally, however, it appears to be stored actually within the bodies of the bacteria. The chromogenic bacteria are therefore classified, in accordance with the final destination of the colouring matter they elaborate, into--