l8o PATHOGENIC BACTERIA. 



the fifth, 30; on the sixth, 150; on the seventh, 1000, 

 etc. ; indicating that the serum exerted a destructive 

 action upon some but not all of the bacteria, and that 

 this power disappeared after the lapse of a certain time, 

 allowing the bacteria to develop ad libitum. 



When the germicide to be studied is a gas, as in the 

 case of sulphurous acid or formaldehyd, a different 

 method must, of course, be adopted. 



It may be sufficient simply to place a few test-tube cul- 

 tures of various bacteria, some with plugs in, some with 

 plugs out, in a closed room in which the gas is afterward 

 evolved. The germicidal action is shown by the failure 

 of the cultures to grow upon transplantation to fresh cul- 

 ture-media. This crude method may be supplemented 

 by an examination of the dust of the room. Pledgets 

 of sterile cotton are rubbed upon the floor, washboard, 

 or any dust-collecting surface present, and subsequently 

 dropped into culture-media. Failure of growth under 

 such circumstances is very certain evidence of good dis- 

 infection. These tests are, however, very severe, for in 

 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 sporogenous 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 ordi- 

 nary pathogenic organisms. 



More refined is the method of saturating sterile sand 

 or fragments of blotting-paper or absorbent cotton 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 

 i in mediately after the experiment. A piece of cotton or 

 blotting-paper or a little sand transferred to fresh culture- 

 media will not give any growth where the disinfection has 

 been thorough. By transplanting from different depths, 

 the sand may be used incidently to show to what depth 

 the gas is capable of penetrating. 



