STEEILIZA TION B Y HEAT. 57 



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 pres- 

 sure 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 thought to occur, and some think 

 the reaction undergoes a change. In the experience 

 of those who have used steam under pressure, not ex- 

 ceeding one or one and one-half atmospheres for ten 

 to fifteen minutes, these obstacles have rarely been 

 encountered. 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 sterilized 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 vio- 

 lently, 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 this that the tempera- 

 ture and time of exposure of articles sterilized by this 

 process cannot usually be controlled with accuracy. 

 It requires some time to reach a given pressure after 



