12 GENERAL MICROBIOLOGY 



in the superheated steam is carried on in a special apparatus 

 called an autoclav, which may be so constructed as to run 

 by direct or indirect steam. The latter is the more desir- 

 able for the sterilization of media. 



V. Sterilization by Light. Light seems to act by pro- 

 ducing powerful chemical germicides, probably organic per- 

 oxides, in the medium surrounding the bacteria. Certain 

 rays of light, the blue, violet and ultraviolet in particular 

 are destructive to living cells. It is to these rays that sun- 

 light owes its disinfecting action. Practical use has been 

 made of the ultraviolet rays in water sterilization by 

 employing the Cooper-Hewitt mercury vapor lamp having 

 a quartz instead of a glass tube, as these rays do not pass 

 through glass. 



VI. Sterilization by Filtration. Sterilization may be 

 effected by the filtration of gases or liquids through 

 materials which will retain microorganisms. 



The best example of the filtration of gases is the use 

 of cotton plugs in flasks and tubes containing microorgan- 

 isms. The cotton is porous enough to allow the necessary 

 interchange of gases but will allow neither dust nor foreign 

 microorganisms to enter. The sterilization of air or other 

 gases if fore 3d through cotton would depend upon the 

 thickness of the cotton layer and also upon the force which 

 was exerted. 



Certain fluids used in bacteriological work cannot be 

 subjected even to a moderate amount of heat without pro- 

 foundly altering their nature. In order to make such a 

 fluid sterile, it is passed through a cylindrical vessel, closed 

 at one end like a test tube, and made either of porous 

 " biscuit" porcelain, hard burnt and unglazed (Chamber- 

 land filter) or of kieselguhr, a fine diatomaceous earth 

 (Berkefeld filter) and termed a bougie or a candle. 



The pores of the finer filters are so small that while 

 liquids, and solids in solution pass through, microorganisms 

 are retained and the liquid passes through in a germ-free 



