TO DETERMINE THERMAL DEATH-POINT. 181 



Easier of execution, but rather more severe, is a 

 method in which cover-glasses are employed. A num- 

 ber of them are spread with cultures of various bacteria, 

 allowed to dry, and then exposed to the gas as long as 

 required. The cover-glasses are afterward dropped into 

 culture-media to permit the growth of the germs not 

 destroyed. 



Animal-experiments may also be employed to deter- 

 mine whether or not a germ that has survived exposure to 

 the action of reagents has its pathogenic power destroyed. 

 An excellent example of this is seen in the case of the 

 anthrax bacillus, a virulent form of which will kill rab- 

 bits, but after being grown in media containing an 

 insufficient amount of a germicide to kill it will often 

 lose its rabbit-killing power, though still able to fatally 

 infect guinea-pigs, or may lose its virulence for both 

 rabbits and guinea-pigs, though still able to kill white 

 mice. 



