In order that any organism may thrive there are a number of general environmental conditions which must be fulfilled. These conditions vary more or less for each kind of organism. Bacteria are no exception to this general rule. These conditions may be conveniently considered under the general heads of _moisture_; _temperature_; _light_; _oxygen supply_; _osmotic pressure_; _action of electricity_; of _Röntgen_ and _radium rays_; _pressure_; _mechanical vibration_; and _chemical environment_, including the _reaction of the medium_, _the effect of injurious chemicals_, and especially the _food requirements of bacteria_. For each of these conditions there is a _maximum_, meaning the greatest amount of the given condition which the organism can withstand, a _minimum_, or the least amount, and an _optimum_ or that amount which is most favorable for development. Further, there might be distinguished a maximum for _mere existence_ and a lower maximum for _development_; also a minimum for _mere existence_ and a higher minimum for _development_. These maxima, minima, and optima for bacteria have been determined with exactness for only a very few of the general conditions and for comparatively few kinds. MOISTURE. The _maximum_ moisture is absolutely pure water, and no organism can thrive in this alone owing to the factor of too low osmotic pressure and to the further factor of absence of food material. There are many bacteria which thrive in water containing only traces of mineral salts and a large class whose natural habitat is surface water. These “water bacteria” are of great benefit in the purification of streams. They are as a class harmless to men and animals. Some of the disease-producing bacteria like _Bacterium typhosum_ (of typhoid fever) and _Vibrio choleræ_ (of Asiatic cholera) were undoubtedly originally water bacteria, and it is rather striking that in these diseases conditions are induced in the intestine (diarrheas) which simulate the original watery environment. The _minimum_ moisture condition is absolute dryness, and no organism can even exist, not to say develop, in such a condition since water is an essential constituent of living matter. Some bacteria and especially most spores may live when dried in the air or by artificial means for months and even years, while some are destroyed in a few hours or days when dried (typhoid, cholera, etc.). The optimum amount of moisture has not been determined with any great accuracy and certainly a rather wide range in percentage of water is permissible with many, though a liquid medium is usually most favorable for artificial growth. The “water bacteria” have been mentioned. In the soil a water content of 5 to 15 per cent. seems to be most suitable for many of the organisms which aid in plant growth. In animals and man the organisms infecting the intestinal tract prefer a high percentage of moisture as a rule, especially those causing disease here. Those found