STERILIZA TION B Y HE A T. 57 



altered. Gelatiii 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 exceeding one 

 or one and one-half atmospheres for a few minutes, ten 

 to fifteen, 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 while the steam 

 that is within it is under pressure, the escape of steam 

 will be so rapid that all fluids within the chambers, 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 steam gauge indi- 

 cates that pressure no longer exists within the chambers, 

 and even then it should be opened very cautiously. It 

 is patent from this that the time and temperature of ex- 

 posure of articles sterilized by this process cannot usually 

 be controlled with accuracy. It requires some time to 

 reach a given pressure after the apparatus is closed, and 

 it abo requires time for cooling down after the desired 

 exposure to such pressure before the apparatus can be 

 opened. 



It is manifest that during these three periods, viz., 

 (a) reaching the pressure desired, (6) time during which 

 the pressure is maintained, and (c) time for fall of press- 

 ure before the chamber can be opened, it is difficult to 

 say certainly to what temperature and pressure the arti- 

 cles in the apparatus have, on the whole, been subjected. 



