BACTERIOLOGY. 6i 



plete drying practically kills all pathogenic bacteria. 

 This statement, however, is only true where complete 

 drying is accomplished, a matter that we can never be 

 sure of unless we control the situation as is done in the 

 laboratory. 



Besides sun-light and drying, ozone 

 Ozone. (nascent oxygen) also takes part in the 



destruction of bacterial life. But as this 

 substance as formed in nature occurs only in small 

 quantities, its importance in this respect is difficult to 

 measure. More ozone is found in the country than in 

 cities, a factor that undoubtedly contributes to the 

 greater healthfulness of rural surroundings. 



Cold, we have already learned, is unfavor- 

 CoLD. able to the growth of bacteria. At the 



freezing temperature (32° F., 0° C.) most 

 bacteria succumb. However, some species exhibit con- 

 siderable resistance to freezing. This is true of the 

 micro-organisms of typhoid fever (typhoid bacillus) 

 and Asiatic cholera (spirillum of Asiatic cholera), 

 epidemics of both of which diseases have been traced 

 to ice as the source of the infection. 



Dilution, or the diminution by dilution, of 



Dilution, the number of bacteria per volume of 



material in which they are contained, is 



also accomplished by nature. How great a benefit 



this is to man will be better explained in a succeeding 



