320 APPLICATION OF METHODS OF BACTERIOLOGY 



as follows: under a sterilized bell-glass of known capacity 

 the test-objects are placed. Into the chamber is then 

 admitted sufficient of a mixture of air and the gas under 

 consideration, of known proportions, to displace com- 

 pletely all the air; or the pure gas itself may be intro- 

 duced in amount necessary to give the desired dilution 

 when mixed with the air in the chamber. At the expiration 

 of the time decided upon for the test the infected articles 

 are removed and the vitality of the bacteria upon them is 

 determined. 



In the case of vapors of volatile fluids, such, for instance, 

 as formalin, the fluid is placed under the bell-glass in an 

 open dish; in another open dish the test-objects are placed. 

 The bell-glass is then sealed to an underlying ground-glass 

 plate by vaselin or paraffin, and the fluid is allowed to 

 vaporize at ordinary room-temperature. The point here 

 to be decided is the volume or weight of such a fluid that 

 it is necessary to expose in an air-chamber of known cubic 

 capacity in order that bacteria may be destroyed by its 

 vapor in a given time. 



In determining the germicidal value of different chemical 

 agents for certain pathogenic bacteria susceptible animals 

 are sometimes inoculated with the organisms after they have 

 been exposed to the disinfectant. If no pathological con- 

 dition results, disinfection is assumed to have been suc- 

 cessful; while if the condition characteristic of the activities 

 of the given organism in the tissues of this animal appears, 

 the reverse is the case. The objections to this method are: 

 "First. The test-organisms may be modified as regards 

 reproductive activity without being killed; and in this 

 case a modified form of the disease may result from the 

 inoculation, of so mild a character as to escape observation. 



