120 DISINFECTION— STERILIZ A TION— DISINFECTANTS 



develop and are killed on the third day. Occasionally a 

 fourth boiling is necessary. It is also true that repeated 

 heating and cooling are more destructive to bacteria than 

 continuous heating for the same length of time, but the 

 development of the spores is the more important factor. 

 Discontinuous heating may also be used at temperatures 

 below the boiling-point for the sterilization of fluids like 

 blood serum, which would be coagulated by boiling. In 

 this case the material is heated at 55° to 56° for one hour, 

 but on each of seven to ten successive days. The inter- 

 mittent heating and cooling is of the same importance as 

 the development of the spores in this case. (Better results 

 are secured with such substances by collecting them asep- 

 tically in the first place.) 



Fig. 72. — The Arnold steam sterilizer for laboratory use. 



Steam.— Stesim is one of the most commonly employed 

 agents for sterilization and disinfection. It is used either 

 as "streaming steam" at air pressure or confined under 

 pressure so that the temperature is raised. For almost all 

 purposes where boiling is applicable streaming steam may 



