DISINFECTION, STERILIZATION AND ANTISEPSIS. 245 



have the germicide in contact 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 substance being tested with the formation of 

 inert or insoluble compounds. In this "inhibition-fiask" the 

 strength of the solution should be such that molecular propor- 

 tions of the chemical are present in sufficient quantity to com- 

 bine with all the germicides carried over. The inhibition-flask 

 is shaken for thirty 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 1 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 determined that 

 the inhibiting chemical itself has no injurious effect on the 

 bacteria. 



The inhibiting chemical must be determined for each in- 

 dividual case. For salts of the heavy metals ammonium 

 sulphide answers well; for mercury-salts, stannous chloride 

 may be used; for formaldehyde, ammonium hydrate; for car- 

 bolic acid, sodium sulphate. 



The testing of gaseous disinfectants, such as sulphur dioxid 

 and formaldehyde, should be conducted under conditions as 

 nearly identical with those met with in actual practice as pos- 

 sible. The test-organisms may be exposed on threads or 

 glass rods, and acted upon by a known volume of strength of 

 germicide for a known length of time. Subsequent treatment 

 of the organisms with a suitable inhibitor is necessary when 

 possible and should growth occur in the cultures following, the 

 test-organism should be identified in order to exclude possible 

 contamination by extraneous organisms. 



In determining the value of germicides for sterflizing liga- 

 tures, the students can apply methods based on the foregoing 



