84 METHODS USED IN THE STUDY OF BACTERIA 



more than three exposures, and to maintain the temperature 

 for a period of time considerably above twenty minutes (one 

 hour or more). 



Disinfection. Chemical substances are used in the 

 laboratory for the purpose of destroying old cultures, for 

 washing the desks, and covering up infectious matter acci- 

 dentally spilled, washing the hands, etc. The chemicals 

 usually used are carbolic acid (3 to 5 per cent solution), 

 corrosive sublimate (i to 1000). The value of these is dis- 

 cussed in a later chapter (see Chap. XI). 



A few disinfectants have been used for sterilizing culture 

 media, such as chloroform and hydrogen peroxide. These 

 chemical substances are later either driven off by heat, or are 

 rendered harmless by the natural changes which go on, so that 

 the nutritive properties of the substances are not destroyed. 



