Sfriiclur< . Growth and Distribution. 11 



number of months. For this reason ice used in cooling 

 drinking water should come only from uncontami- 



nated sources. 







Effect of heat on bacteria. All forms of life are de- 

 stroyed by high temperatures. The seeds of the higher 

 plants are not so easily killed by heat as are the plants 

 themselves and in the same way the spores formed by 

 certain of the bacteria are very difficult to kill. Some 

 must be subjected to the temperature of boiling water 

 (212 F.) for hours in order to destroy them. 



Two forms of heat are used to destroy bacteria, moist 

 heat, as steam or hot water, and dry heat as produced 

 in an oven. As every one knows, one can place his 

 hand in an oven heated much above 212 F. without in- 

 jury, but the same exposure to steam or boiling water 

 would result in a serious burn. The effect on the bac- 

 teria is the same. For this reason when it is desired to 

 destroy germ life on or in any substance, moist heat is 

 preferably used if it will not injure the object. 



Effect of light on bacteria. All the plants which con- 

 tain the green coloring matter, chlorophyll, can produce 

 a normal growth only in the presence of light. Fungi 

 grow rapidly in the dark. To some of the fungi the 

 light is not especially injurious. The bacteria, espe- 

 cially the pathogenic bacteria, even in a spore stage, 

 are easily killed by direct sunlight. As previously 

 stated, the spores are very difficult to kill by heating, 

 much more so than the vegetating cell, but sunlight 

 kills the spores of most forms almost as quickly as it- 

 does the growing cell. 



While the direct sunlight is very efficient in destroy- 

 ing both cells and spores, the diffused light of an ordi- 



