METHODS OF TESTING DISINFECTANTS. 519 



gas under consideration to eliminate completely all the 

 air; or, the pure gas itself may be introduced in the 

 amount necessary to give the desired dilution when 

 mixed with the air in the chamber. After the time de- 

 cided upon for the test the infected articles are removed 

 and the vitality of the bacteria upon them is determined. 



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

 instance, as formaline, 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 vaseline or paraffin, 

 and the fluid is allowed to vaporize at ordinary room 

 temperature. The point here to be decided is the vol- 

 ume 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 upon certain pathogenic bacteria, sus- 

 ceptible animals are sometimes inoculated with the 

 organisms after they have been exposed to the disin- 

 fectant. If no pathological condition results, disinfec- 

 tion is presumed to have been successful; 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 

 that have been raised are : "First. The test-organisms 

 may be modified as regards reproductive activity with- 

 out 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. Second. An ani- 

 mal that has suffered this modified form of the disease 

 enjoys protection, more or less perfect, from future at- 



