STERILIZATION. 5 1 



may occasionally become very intense. It is well, if possible, 

 to have it fastened to a brick wall. 



Boiling. — Boiling is an efficient method of sterilization. 

 It is often used for instruments. In laboratory work steam 

 is generally substituted for it. 



Steam Sterilization. — Steam steriHzation is the most 

 generally used of all forms of sterilization and is the most 

 effective. It is employed for perishable bodies which would be 

 injured by dry air steriHzation or by chemical germicides; for 

 example, it is used for surgical instruments and for culture- 

 media; in laboratory work, especially for culture-media. It 

 has been found that there are some forms of bacteria which, 

 in the resting or spore stage, can resist even the action of steam 

 for several hours. Such prolonged exposure to steam would 

 be very injurious to culture-media, which are more or less un- 

 stable organic substances. What is called fractional, inter- 

 mittent or discontinuous sterilization is used for such materials. 

 By that plan the medium is sterilized with steam for fifteen 

 minutes on each of three consecutive days. The object of inter- 

 mittent steriHzation as explained by Tyndall, who proposed it, 

 is this : The culture-medium may be supposed to contain fully 

 developed bacteria, and also bacteria in the spore or resting 

 stage. The first sterilization of fifteen minutes will probably be 

 sufficient to destroy all the fully developed bacteria; during the 

 twenty-four hours between the first and second steriHzation all 

 of the spores which have survived the first steriHzation may be 

 expected to have become fully developed into bacteria which 

 can be destroyed by the second steriHzation; the third steril- 

 ization is directed against any spore forms which may possibly 

 have survived the second steriHzation. 



Although the spore forms which are so extremely resistant 

 are non-pathogenic, as for example spores of the hay bacillus 

 and of the potato bacillus, they nevertheless are capable of 

 ruining the culture-media with which one works. 



