PHYSICAL DISINFECTING AGENTS 257 



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. 



DiSINFFXTING 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 



