7V) nRTKRMLVK THERMAL DKATH-PO/NT. 181

Easier of execution, but rather more severe, is a
method in which cover-classes 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-classes 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.