242 POPULAR SCIENCE MONTHLY. 



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 killea 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 weak- 

 ened that they will no longer produce infection. The germicidal 

 action of the sun's rays is most marked at the blue end of the spec- 

 trum, at the red end there is little or no germicidal action. It is 

 evident that the continuous daily action of the sun along with desic- 

 cation are important physical agents in arresting the further develop- 

 ment 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 con- 

 nection 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 organ- 

 isms, 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-nroducing bacteria belong to this adaptive 

 class. When a pigment-producing organism is grown without free 

 oxygen its pigment production is almost always stopped. For anae- 

 robic forms N" and H= give the best atmosphere for their growth, 

 whilst CO.' 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 multiplica- 

 tion of bacteria in water under natural conditions are a low altitude, 

 warmth, abundance of organic matter and a sluggish or stagnant con- 

 dition of the water. As regards water-borne infectious diseases, such 



