394 AGRICULTURAL BACTERIOLOGY. 



While this is extreme it is a universal fact that dry heat is 

 very inefficient in destroying bacteria. Hence it is difficult to 

 destroy the bacteria in dust or upon the dry walls of rooms, 

 by simple heat. 



Moist heat destroys much more quickly. While some bac- 

 teria will resist, for a time, a temperature somewhat above 

 boiling, most of them are rapidly destroyed in boiling water. 

 A temperature of a few degrees above boiling, obtained by 

 steam heated under pressure, is sufficient to destroy all life, 

 even the most resisting spores. The exact temperatures vary 

 widely for different bacteria, but a temperature of 235 F., 

 continued for fifteen minutes, is sufficient to destroy all bac- 

 terial life. It must be remembered that such high tempera- 

 tures are not needed for destroying most bacteria, and, with 

 the exception of the anthrax and tetanus bacillus, all of the 

 bacteria which commonly cause diseases among man and our 

 domestic animals, are destroyed by a temperature of boiling, 

 and commonly at a temperature much lower. 



This is a matter of great practical importance. It teaches 

 that it is not necessary to use such high temperatures as ob- 

 tained by superheated steam, in order to disinfect material in- 

 fected with the bacteria of common diseases. Cholera, for 

 example, is killed by a temperature of 156 F., if continued 

 for half an hour, and the same is true of typhoid fever and 

 diphtheria. Tuberculosis is, probably, at least rendered harm- 

 less by a temperature of 140 F., if continued for half an hour 

 under proper conditions. 



Sunlight. Sunlight is a very effectual germicide. Bacteria 

 exposed to direct sunlight are killed in a few hours, and even 

 diffused daylight has a marked injurious effect upon their 

 vitality. The brighter the light the more efficacious its action. 

 While sunlight is thus an exceptionally good germicide, its 

 practical value in disinfection is not very great because of the 

 difficulty of its application. To disinfect an object by sunlight 



