306 Value of Antiseptics 



the cultures there are immense numbers of bacteria in the 

 deeper portions of the bacterial mass upon which the gas has 

 no opportunity to act, and in the dust there are many spor- 

 ogenous organisms of extreme resisting power. Failure to 

 kill all the germs exposed in such manner is no indication 

 that the vapor cannot destroy all the ordinary pathogenic 

 organisms. 



A more refined method of making the tests consists in 

 saturating strips of blotting-paper, absorbent cotton, various 

 fabrics, etc., with cultures and exposing them, moist or dry, 

 to the action of the gas. Such materials are best made 

 ready in Petri dishes, which are opened immediately before 

 and closed immediately after the experiment. If, when 

 transferred to fresh culture media, the exposed objects fail 

 to give any growth, the disinfection has been thorough. If 

 the penetrating power of a gas, such as formaldehyd, is to 

 be tested, it can be done by inclosing the infected paper or 

 fabrics in envelopes, boxes perforated with small holes, 

 tightly closed pasteboard boxes, and by wrapping them in 

 towels, blankets, mattresses, etc. 



Easier of execution, but rather more severe, is a method 

 in which cover-glasses are employed. A number of them 

 are sterilized, spread with cultures of various bacteria, al- 

 lowed to dry, and then exposed to the gas as long as re- 

 quired. They are subsequently dropped into culture media 

 to permit the growth of the organisms not destroyed. 



Animal experiments may also be employed to determine 

 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 rabbits, 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. 



