METHODS OF TESTING DISINFECTANTS. 661 



chambers. The object is to determine the proportion 

 of the gas, when mixed with air, that is required to 

 destroy the bacteria exposed to its action in a given 

 time. For this purpose the test is usually made 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 vol- 

 ume cr 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 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 



