STERILIZATION — ANTISEPSIS — FOOD PRESERVATION 63 



tance, even though it does not insure complete sterilization. 

 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 atmos- 

 phere. It also operates to remove a large proportion of the 

 bacteria from • drinking water when stored in suitable reservoirs. 

 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 filters 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 example 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 intelligence and care, and 

 when carelessly employed they are worse than useless. 



Desiccation 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 



