Influence of Sunlight 71 



demonstrated that a number of soil bacteria and also 

 of water bacteria injuriously affected the typhoid germs, 

 and that such injurious effects were not observed when 

 the typhoid germs were alone. Furthermore, the in- 

 jurious action was found to be dependent to a great 

 extent on the temperature to which the bacteria were 

 exposed. 



Sunlight. — This is another factor which undoubtedly 

 plays some part in the increase or decrease of bacteria 

 in water. Reference has been made to the germicidal 

 action of direct sunlight, and it has been pointed out 

 that in the deeper layers of water such action- can be 

 of only small moment. It is still uncertain to what 

 extent direct sunlight actually contributes to the de- 

 crease of bacteria in water. There can be scarcely any 

 doubt, however, that during the long hours of bright 

 sunshine in the summer months, and, particularly in 

 shallow bodies of water, the germicidal action must be 

 of considerable significance. Furthermore, some con- 

 sideration rhust also be given to the indirect action of 

 sunlight in the development of the algae. The latter 

 withdraw in their growth considerable quantities of 

 nitrates and other food material from the water, and 

 thereby render the conditions less favorable for the 

 development of the bacteria. 



Animalcules injurious to bacteria. — The number of 

 bacteria in water is likewise affected by certain animal- 

 cules that use them as food. It is interesting to watch 

 these little organisms which, though giants in size as 

 compared with the bacteria, are still so small as to 

 require magnification by a powerful microscope leng 



