STERILIZATION IN STEAM AT 100 C. 7 



(d) Light the gas. It is well to hold a lighted taper to the burner before 

 turning on the tap, since if gas escape it will mix with the air between the 

 inner and outer walls of the sterilizer and so tend to cause an explosion. 



(e) Regulate the flame so that the temperature rises slowly ; this is par- 

 ticularly important if the sterilizer contain vessels of thick glass, e.g. test- 

 tubes on feet, glass dishes, etc. 



(/) When the thermometer records a temperature of 175-180 C. in the 

 interior of the sterilizer, lower the gas gently, leaving sufficient flame to 

 maintain the temperature at 180 C. or thereabouts for half an hoar or so. 



With a little practice this is easily done. Rather than use the fingers it is better 

 to manipulate the tap by tapping it with some heavy instrument such as the spanner 

 used for tightening the bolts of the autoclave, which will give very delicate control 

 over the supply of gas, and will obviate the annoyance caused by accidentally 

 turning out the gas altogether. 



When experience has been gained, a thermometer can be dispensed with; at 

 a temperature of 180 C. wool and paper become slightly scorched, and when 

 this effect is noted the gas is turned down. 



(g) When sterilization is completed turn out the gas, but allow the tem- 

 perature to fall considerably before removing the contents, because glass, 

 and especially thick glass, is liable to crack if exposed to a sudden change 

 of temperature. 



[With Hearson's hot air sterilizer the procedure is the same, except that stage 

 (/) is omitted ; when the temperature for which the capsule is set is reached, the 

 gas is automatically lowered. It is only necessary therefore to note when the 

 thermometer reaches the point at which sterilization is to be effected, and half 

 an hour later to turn out the gas and proceed as in (g).] 



SECTION II. STERILIZATION BY MOIST HEAT. 



Sterilization by moist heat may be effected in one of three ways. 



1. By heating in water or steam at 100 C. 



2. By heating in steam under pressure. 



3. By discontinuous heating at low temperatures. 



1. Sterilization in steam at 100 C. 



Simple boiling or exposure to steam at 100 C., even though the exposure 

 be prolonged, is not a reliable method of sterilization. 



When micro-organisms have been dried, their resistance to the effects of heat 

 is much enhanced, and especially is this the case when they are mixed with sub- 

 stances of an albuminoid nature. Further there are certain resistant forms of 

 bacterial protoplasm known as spores, which in the majority of cases at least 

 are not destroyed by heating to 100 C., even when the temperature is maintained 

 for several minutes. 



In France sterilization by moist heat at 100 C. is very seldom employed, 

 except for sterilizing syringes for inoculation. In this case a sufficient 

 degree of asepsis is obtained by boiling in water for 15 to 20 minutes at 

 ordinary atmospheric pressure. 



[In England on the other hand, and] in Germany, sterilization by moist 

 heat at 100 C. is in general use. The operation is carried out in a Koch's 

 sterilizer or steamer, and must be repeated at intervals of 24 hours on at 

 least two, but ordinarily on three, successive days. 



This method is the outcome of an observation by Tyndall to the effect that 

 while it is impossible to sterilize an infusion of hay by boiling it continuously even 

 for a prolonged period, yet by boiling it for a short time on three successive days 

 all living organisms are destroyed. 



