DISINFECTANTS AND ANTISEPTICS. 197 



we are indebted for demonstrating its accuracy and "practi- 

 cability. 



Method. To 15 c.c. of sterile water in a 60 c.c. Erlen- 

 meyer flask add 2 c.c. of a virulent culture of the test- 

 organism. Then add a solution of the substance under in- 

 vestigation in the proportion necessary to give the dilution 

 wished. Mix thoroughly, and allow this " action-flask " to 

 stand as long as it is desired to have the germicide in con- 

 tact with the test-organism (action-period). Transfer 0.5 

 c.c. from the action-flask to a flask containing 200 c.c. of a 

 solution of some chemical capable of decomposing the sub- 

 stance being tested with the formation of inert or insoluble 

 compounds. In this " inhibition-flask " the strength of the 

 solution should be such that molecular proportions of the 

 chemical are present in sufficient quantity to combine with 

 all the germicide carried over. The inhibition-flask is 

 shaken for 30 seconds, and i c.c. transferred from it to 100 

 c.c. of sterile water in another, the " dilution-flask." After 

 two minutes, three agar tubes are inoculated with i c.c. each 

 from the dilution-flask, plated, and growth watched for. 



Control-experiments should be performed to determine 

 that the dilution of the test-culture is not too great when 

 carried through the three flasks. It likewise should be de- 

 termined that the inhibiting chemical has no effect on the 

 bacteria. 



What the inhibiting chemical shall be must be deter- 

 mined for each individual case. For salts of the heavy 

 metals ammonium sulphide answers well ; for mercury salts, 

 stannous chloride may be used; for formaldehyde, ammo- 

 nium hydrate; for carbolic acid, sodium sulphate. 



The testing of gaseous disinfectants, such as sulphur 

 dioxide and formaldehyde, must be conducted under condi- 

 tions as nearly parallel to actual practice as possible. The 

 test-organisms may be exposed on threads or glass rods, and 



