STERILIZATION. 55 



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

 to have it fastened to a brick wall. 



Boiling. — Boiling in water is an efficient means of steriliza- 

 tion for some purposes. 



Steam Sterilization.— In laboratory work "live" steam 

 or steam under pressure is generally substituted for simple 

 boiling in the water and is more effective than the latter. 



By "live" steam is meant the steam which streams off from 

 the Koch or from the Arnold sterilizer described below. By 

 steam under pressure is meant that generated in an autoclave 

 also described below. But in both cases the steam to be ef- 

 fective must be saturated with moisture. In the Koch and in 

 the Arnold sterilizers this is always the case; though experi- 

 ments have been made in which the steam was kept at ioo° C, 

 but deprived of moisture, and in this case it was much less ef- 

 fective. It becomes under such circumstances merely air with 

 more or less moisture, and experiments have shown that the 

 sterilizing power of heated air is in proportion to the amount 

 of moisture contained. Steam under pressure is not neces- 

 sarily saturated with moisture. If water is boiled in a closed 

 vessel the steam will be mixed with the air which is inclosed 

 unless this is allowed to escape before the vessel is closed. This 

 matter is of practical importance in using the autoclave. 



Steam is employed for perishable bodies which would be 

 injured by dry- air sterilization 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 the action of steam 

 even for several hours. Such prolonged exposure to steam 

 would be 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 



