THE DESTRUCTION OF BACTERIA 



69 



over. This method is used in laboratories for the sterilization of Petri 

 dishes, flasks, test tubes, and pipettes, and for articles which may be in- 

 jured by moisture. Both heating and subsequent cooling should be done 

 gradually to avoid cracking of the glassware. 



Moist Heat. Instruments, syringes, and other suitable objects may 

 be sterilized by boiling in water. Boiling for about five minutes is amply 

 sufficient to destroy the vegetative forms of all bacteria. For the de- 

 struction of spores, boiling for one or two hours is usually sufficient, 

 though the spores of certain saprophytes of the soil have been found 



FIG. 8. HOT AIR STERILIZER. 



occasionally to withstand moist heat at a temperature of 100 C. for 

 as long as sixteen hours. 1 The addition of 1 per cent of sodium car- 

 bonate to boiling water hastens the destruction of spores and prevents 

 the rusting of metal objects sterilized in this way. The addition of car- 

 bolic acid to boiling water in from 2 to 5 per cent usually insures the 

 destruction of anthrax spores, at least, within ten to fifteen minutes. 



Exposure to live steam is probably the most practical of the methods 

 of heat sterilization. It may be carried out by simple makeshifts of 

 the kitchen, such as the use of potato-steamers or of wash-boilers. For 



1 Christen, loc. cit. 



