46 INFECTION AND IMMUNITY 



other kinds of infectious material in which the spe- 

 cific germ has not yet been demonstrated. 



While dry hot air is, as a rule, unreliable for the de- 

 struction of disease germs, certain bacteria are quickly 

 destroyed by desiccation. This is true of the cholera 

 spirillum and of the micrococcus of pneumonia. On 

 the other hand, the bacillus of typhoid fever, the bacil- 

 lus of diphtheria, the bacillus of tuberculosis and the 

 bacillus of bubonic plague may retain their vitality for 

 weeks, or even months, when in a desiccated condi- 

 tion. This is true also of the virus of smallpox and 

 of scarlet fever. 



Low temperatures do not destroy bacteria. They 

 have been exposed to a temperature of 87 C., ob- 

 tained by the evaporation of liquid carbonic acid, 

 but when again brought under favourable conditions 

 showed no diminution in their capacity for develop- 

 ment. Repeated freezing and thawing has, however, 

 a deleterious action. The typhoid bacillus may be 

 killed in cultures which are frozen and thawed out at 

 intervals of three days, by repeating the operation 

 five or six times. 



The facts stated in this chapter make it evident 

 that heat constitutes the most generally useful agent 

 for the destruction of infectious material. Any arti- 

 cle of food or drink which has been recently brought 

 to a temperature approaching that of the boiling- 



