200 UNICELLULAR PLANTS. 



may be effected by heat and continued, after cooling, by exclusion of 

 germ-laden air. Disinfection, which is the destruction of bacterial life by 

 powerful poisons, is another form of sterilization. Still another is filtra- 

 tion through media impervious to germs, such as occurs in the well- 

 known clay, or porcelain, water-filters. In the last case the pores of the 

 filter are large enough to allow the water very slowly to pass, but too small 

 for the bacteria. 



In some cases, especially those in which disease-producing (pathogenic} 

 germs may be present and yet it is impossible to use poisons and undesira- 

 ble to use a high temperature, Pasteurization is resorted to. This con- 

 sists in heating to a temperature (usually 75 C.) high enough to destroy 

 the particular pathogenic germs supposed to be present, but not high 

 enough to alter the digestibility or other valuable properties of the liquid 



in question. 



For the medical, economic, and sanitary aspects of problems relating 

 to the bacteria, reference must be had to the numerous treatises upon 

 Bacteriology, perhaps the youngest, and certainly one of the most impor- 

 tant, of the biological sciences. 



