half an hour to two hours; in the diphtheria bacillus in half an hour to one hour, and to the tubercle bacillus in a few minutes to several hours. Even anthrax spores are killed by direct light in three and a half hours. Diffuse light is also injurious, though its action is slower. By exposing pigment-producing bacteria to sunlight colorless varieties can be obtained, and virulent bacteria so weakened that they will no longer produce infection. The germicidal action of the sun's rays is most marked at the blue end of the spectrum, at the red end there is little or no germicidal action. It is evident that the continuous daily action of the sun along with desiccation are important physical agents in arresting the further development of the disease germs that are expelled from the body.
It has been shown that sunlight has an important effect in the spontaneous purification of rivers. It is a well-known fact that a river, despite contamination at a given point, may show little or no evidence of this contamination at a point further down in its course. Buchner added to water 100,000 colon bacilli per cubic centimeter, and found that all were dead after one hour's exposure to sunlight. He also found that in a clear lake the bactericidal action of sunlight extended to a depth of about six feet. Sunlight must, therefore, be taken into account as an agent in the purification of waters, in addition to sedimentation, oxidation and the action of algae.
Air or the oxygen it contains has important and opposite effects on the life of bacteria. In 1861, Pasteur described an organism in connection with the butyric acid fermentation which would only grow in the absence of free oxygen. And since then a number of bacteria, showing a like property, have been isolated and described. They are termed anaerobic bacteria, as their growth is hindered or stopped in the presence of air. The majority of the bacteria, however, are aerobic organisms, inasmuch as their growth is dependent upon a free supply of oxygen. There is likewise an intermediate group of organisms, which show an adaptability to either of these conditions, being able to develop with or without free access to oxygen. Preeminent types of this group are to be met with in the digestive tract of animals, and the majority of disease-producing bacteria belong to this adaptive class. When a pigment-producing organism is grown without free oxygen its pigment production is almost always stopped. For anaerobic forms N and H2 give the best atmosphere for their growth, whilst CO2 is not favorable, and may be positively injurious, as, e. g., in the case of the cholera organism.
The physical conditions favoring the presence and multiplication of bacteria in water under natural conditions are a low altitude, warmth, abundance of organic matter and a sluggish or stagnant condition of the water. As regards water-borne infectious diseases, such