STERILIZATION ANTISEPSIS FOOD PRESERVATION 63 



The process removes not only many of the bacteria, but also 

 much other material which serves to protect them and even to_ 

 furnish food for their development. Another mechanical method 

 is that of comminution, actual crushing of the bacterial cells. 

 It is of very narrow application and not to be relied upon. 

 High pressures have been employed to destroy bacteria, but 

 hydrostatic pressure of even 1000 atmospheres does not produce 

 complete sterilization. Sedimentation is a method of primary 

 importance, especially in the removal of suspended bacteria from 

 the atmosphere. It also operates to remove a large proportion 

 of the bacteria from drinking water when stored in suitable reser- 

 voirs. Filtration of fluids is an important means of sterilizing 

 them. Air may be sterilized by drawing -it slowly through a 

 sufficient layer of cotton. Water becomes bacteria-free as it 

 niters through the soil, so that waters from the depths of the earth 

 are sterile. Liquids are commonly sterilized in the laboratory 

 by forcing them through a layer of unglazed porcelain (Pasteur- 

 Chamberland filter) or through a compact wall of diatomaceous 

 earth (Berkefeld filter). Liquids rich in bacteria, such for ex- 

 ample as cultures in broth, may be rendered bacteria-free in this 

 way. These filters have also been employed for the sterilization 

 of drinking water, but their use for this purpose requires intelli- 

 gence and care, and when carelessly employed they are worse 

 than useless. 



Dessication is destructive to many microbes, especially those 

 which do not form spores. The germs of Asiatic cholera are dead 

 in a few hours after complete drying. The spores of the anthrax 

 bacillus on the other hand remain alive for at least ten years after 

 drying. Most bacteria resist drying long enough so that they 

 may be transferred by air currents as dust and still be capable of 

 growth. 



Light is injurious to bacteria and direct sunlight is rapidly 

 fatal to them, even in spore form. Light seems to act by pro- 

 ducing powerful chemical germicides, probably organic peroxides, 

 in the medium surrounding the bacteria. Such substances are 



