STERILIZATION 



9 



instantaneously, and spores in from five to fifteen minutes 

 ordinarily, although many spores of resistant species are not 

 killed by several hours' heating at 100 C. Water suspected 

 of sewage contamination may thus be rendered safe for 

 drinking purposes simply by boiling for a few minutes. 



This method is applicable to metal instruments, syringes, 

 rubber stoppers, rubber and glass tubing, and other small 

 apparatus. 



(b) Discontinuous Heating. (Tyndall method.) Tyndall 

 observed that certain resistant 

 forms found in an infusion made 

 from hay were not destroyed by 

 heating the infusion at 100 C., 

 once, even when the temperature 

 was sustained for a prolonged 

 period, yet by boiling it for a 

 short time on three successive 

 days all living organisms were 

 destroyed. His theory was that 

 by heating at 100 C., the vege- 

 tative forms but not the spores 

 were killed. The latter germinate 

 as the fluid cools and are killed 

 during the second heating. A 

 few spores, however, escape de- 

 struction at the second heating; 



these will have germinated by the time the third heating is 

 due. After the third heating sterilization is accomplished. 



The explanation now given, however, is that the resist- 

 ance of microorganisms is gradually lowered under the 

 influence of repeated heatings. This principle of heating 

 on three successive days, a medium to be sterilized is now 

 known as the Tyndall method of sterilization. In general 

 laboratory practice, steam is used instead of water at 100 

 C., but this necessitates special apparatus, whereas water 

 lends itself readily to the means at hand. 



FIG. 3. Arnold Steam 

 Sterilizer. 



