STERILIZATION BY HEAT 83 



first method described; but it is allowed to act for a much 

 longer time, usually for not less than an hour; or steam under 

 pressure, and consequently of a higher temperature, is now 

 frequently employed. By the latter procedure a single 

 exposure of fifteen minutes is sufficient for the destruction 

 of practically all bacilli and their spores, providing the 

 pressure of the steam is not less than one atmosphere over 

 and above that of normal; this is approximately equivalent 

 to a temperature of 122 C. to which the organisms are 

 exposed. 



The objection that has been urged to both of these 

 methods, particularly that in which steam under pressure 

 is employed, is that the properties of the media are altered. 

 Gelatin is said to become cloudy and lose the property of 

 solidifying; in bouillon and agar-agar fine precipitates are 

 said to result, and some believe the reaction undergoes a 

 change. In the experience of those who have used steam 

 under pressure not exceeding one atmosphere for ten to 

 fifteen minutes these obstacles have rarely been encoun- 

 tered. There is one point to be borne in mind, however, in 

 using steam under pressure, viz., it is not possible to regulate 

 the time of exposure to the same degree of nicety as where 

 ordinary live steam is used. The reason for this is that if 

 the apparatus be opened to remove the objects being steril- 

 ized while the steam within it is under pressure, the escape 

 of steam will be so rapid that all fluids within the chamber, 

 thus suddenly relieved of pressure, will begin to boil violently, 

 and, as a rule, will boil quite out of the tubes, flasks, etc., 

 containing them. For this reason the apparatus must be 

 kept closed until cool, or until the gauge indicates that 

 pressure no longer exists within the chamber, and even 

 then it should be opened very cautiously. It is patent from 



