46 METHODS OF STUDYING MICROORGANISMS 



ones. The typhoid and diphtheria organisms may resist 200 

 below zero C. or 300 F., while some of the more delicate 

 organisms quickly die at zero. 



In sterilization that method is chosen that will do the 

 least damage to any object to be conserved. Simple boiling 

 should be undertaken whenever practicable, and immersion 

 for five minutes in boiling water will destroy the vegetative 

 forms of all bacteria. For spores, however, at least of the 



FIG. 20. Laboratory hot-air sterilizer. 



disease-producing kind, two hours is necessary. It is advis- 

 able to add 1 per cent, of sodium carbonate to the water. This 

 assists in killing of spores, and metal objects are not so apt to 

 rust. This simple boiling for ten minutes is sufficient for 

 dry cleansed syringes, trays, dishes, and surgical instruments 

 in the absence of infective material known to contain spores. 

 Sterilization in live steam is the most practical method of 

 killing bacteria, as it can be conducted in the kitchen. In 

 the laboratory it is done by the Arnold sterilizer (Fig. 19). 



