PHYSICAL AND CHEMICAL AGENTS 29 



germicidal effect upon bacteria, many species being 

 killed within a few seconds upon exposure to the 

 direct rays of the sun, and all showing different 

 degrees of sensitiveness to light. The electric light 

 exerts a similar effect to sunlight, but is less powerful. 



Moisture. The absence of moisture may cause 

 the death of bacteria or cause a suspension of de- 

 velopment. Certain species are deprived of the 

 power of reproduction without moisture, which under 

 favorable conditions will again grow and multiply. 



Desiccation (drying) destroys nearly all of the 

 pathogenic bacteria, the tubercle bacillus being 

 one of the most resistant to drying and the cholera 

 spirillum one of the most sensitive. The spores, 

 however, are extremely resistant to drying. It is 

 said that the spores of the anthrax bacillus will 

 survive drying for ten or more years. 



This sensitiveness to drying, shown by a large 

 majority of disease-germs, would indicate that infec- 

 tion through the air, cannot be so common as had 

 been supposed. 



Oxygen. In their relation to oxygen bacteria 

 are divided into three classes : (1) those which re- 

 quire free oxygen for the maintenance of their 

 activities, called obligatory aerobes; (2) those which 

 do not grow except in the almost entire absence of 



