DISINFECTION. 393 



clothing, food, etc., it is best to divide the subject into two 

 divisions: (i) We must learn what agents are efficacious in 

 destroying bacteria. (2) We must learn how these agents 

 may be practically used. It is manifest that not all germicides 

 can be used under all conditions. A violent poison may be a 

 good germicide, but it cannot be used for preserving food. 

 The question of the practical application is of fully as much 

 significance as the actual germicidal properties of different 

 agents. 



The methods of destroying bacterial life are both physical 

 and chemical, each of which has its uses under different con- 

 ditions. 



I. PHYSICAL AGENTS. 



Heat. Heat and light are the only physical agents which 

 are of value in destroying bacterial life. No living things can 

 withstand high heat, and heat is therefore one of the best dis- 

 infectants. The only question is, how high a heat bacteria 

 can withstand, and whether a sufficient heat may be applied 

 for the object to be disinfected. All active bacteria are killed 

 by a moderately high heat, most forms of actively growing 

 bacteria being destroyed by a temperature of about 140 F. 

 A few can live and actually grow at a temperature closely 

 approximating this, but such bacteria are unusually rare, 

 and do not commonly enter into the question of disinfection. 

 But while such moderate temperature destroys most active 

 forms, it is rarely of any use as a disinfectant, since large num- 

 bers of bacteria form resisting spores which can endure a much 

 higher heat. The spores of bacteria commonly resist the 

 temperature of boiling water for a short time, and some species 

 resist this temperature for an hour or more. 



The temperature which will destroy bacteria differs much 



according to the method of application. Dry heat is far less 



efficacious than moist heat. Bacteria spores, when dry, are 



known to resist a temperature of 280 F. for some hours. 



33 



