CULTURAL METHODS 73 



to all parts of the material to be sterilized. The cotton 

 which is used for closing the openings of bottles and flasks 

 is usually slightly browned by this exposure. Care should 

 be used that the temperature does not reach so high a point 

 that the cotton is charred. 



Many objects which will not withstand heating at high 

 temperatures or in a dry atmosphere may be sterilized by 

 long continued or repeated boiling in water, or exposure to 

 streaming steam, that is, to steam at atmospheric pressure. 

 The latter is usually accomplished by the use of an instru- 

 ment termed an Arnold sterilizer which supplies a con- 

 tinuous flow of live steam to the box in which the materials 

 to be sterilized are placed. In such sterilization it is fre- 

 quently customary to heat for fifteen minutes on each of 

 two or three consecutive days. In other cases the material 

 is heated for a longer period at one time. 



Sterilization is most efficiently effected by the use of 

 steam under pressure. Certain types of media cannot be 

 sterilized in this fashion because they may be decomposed, 

 but most materials used in the laboratory in media, can 

 withstand a temperature of 110 to 122 C. secured by 

 steam pressure of eight to fifteen pounds per square inch. 

 The apparatus commonly used is some form of an auto- 

 clave. In the use of the autoclave care should be exercised 

 that all of the air has been driven out by the entering 

 steam before it is tightly closed, otherwise the temperature 

 will not be as high as would be assumed from the reading on 

 the pressure gauge of the instrument. Ten to twenty min- 

 utes exposure at fifteen pounds pressure will destroy any 

 living organism. Where large bulk of material is exposed 

 or where the material is of such a nature that penetration 

 of steam is slow a longer period of time must be used. 



Sterilization by Light. Sunlight destroys microorgan- 

 isms rapidly. The ultra-violet rays of the spectrum are 

 particularly potent. It is possible quite efficiently and sat- 



