256 BACTERIA, YEASTS, AND MOLDS 



be considered: (i) What disinfectants are capable of 

 destroying the bacteria? (2) How can these agents be 

 most practically applied ? It is of course manifest that 

 not all germicides can be used under all conditions. Vio- 

 lent poisons, like corrosive sublimate, might be used in 

 some cases, while it would be out of the question to use 

 them in others. The question, therefore, of the appli- 

 cation of the disinfectants is of even more importance 

 than a knowledge of these antiseptics themselves. 



Disinfecting Agents — Physical 



The physical agencies which destroy microorganisms 

 have already been considered in previous chapters, and 

 a summary only is here needed. They are briefly the 

 following : 



Heat. All active growing forms of bacteria are 

 destroyed by moderate heat. A temperature of 140°, 

 maintained for half an hour, is usually capable of de- 

 stroying them, and a higher temperature quickly kills 

 them. Spores, however, are not killed by a temperature 

 short of actual boiling, and some spores are killed only 

 by prolonged boiling. Moist heat of steam is more effi- 

 cacious than dry heat. Bacteria spores may withstand a 

 dry heat of 280" for some hours, but they cannot with- 

 stand a moist heat of steam that is much above boiling. 



A matter of practical importance is the recognition of 

 the fact that most of our contagious diseases are caused 

 by microorganisms that do not produce spores. Conse- 

 quently lower temperatures than boiling are commonly 

 sufficient for disinfection. The only common disease that 



