STERILIZATION. 63 



may occasionally become very intense. It is well, if pos- 

 sible, 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 sterilization 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 sterilization or by chemical ger- 

 micides ; for example, it is used for surgical instruments and 

 for culture-media; in laboratory work, especially for cul- 

 ture-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 pro- 

 longed exposure to steam would be very injurious to cul- 

 ture-media, which are more or less unstable organic sub- 

 stances. What is called fractional, intermittent or dis- 

 continuous 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 sterilization 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 bac- 

 teria; during the twenty-four hours between the first and 

 second sterilization all of the spores which have survived 

 the first sterilization may be expected to have become fully 

 developed into bacteria which can be destroyed by the 

 second sterilization ; the third sterilization is directed against 

 any spore forms which may possibly have survived the sec- 

 ond sterilization. 



Although the spore forms which are so extremely resist- 

 ant are mostly non-pathogenic, as for example the bacilli of 



