STANDARDIZATION OF DISINFECTANTS 249 



for two dilutions to give practically identical results. Note, for 

 example, such results as these: 



Carbolic acid 1-120 + + 



1-130 + + + 



Sample 1-720 -f- + 



1-780 + + 

 1-840 + + + + 



In this case if no other dilutions of the sample are tested the 

 value is determined by comparing both 720 and 780 with carbolic 

 acid dilute 120 and the result is 6.25, the average of the two 

 values. 



It will be noted that in the foregoing description no mention 

 is made of the temperature at which the test is made. While 

 temperature affects very vitally the process of disinfection, the 

 changes in temperature of an ordinary working room rarely 

 exceed 10 C., while the average change would not exceed 5 C., 

 the year round; and since the standard is affected to practically 

 the same degree by these changes it seems an unnecessary com- 

 plication to carry out the test at a rigidly defined temperature. 



It will also be noted that the time of contact between organism 

 and disinfectant is only one-third as long as is recommended in 

 most tests. While it may form in some cases a better picture of 

 the value of a disinfectant to find its efficiencies at two periods such 

 as 2j^ min. and 15 min., practically no material change in its 

 value results from such a course. The taking of a subculture 

 each minute rather than at 2% min. intervals makes for greater 

 accuracy, but this does not materially affect the results. 



THE WORTH HALE TOXICITY COEFFICIENT 



Worth Hale of the U. S. Public Health Service Hygienic 

 Laboratory has worked out a method for determining the com- 

 parative toxicity of disinfectants of which the following is a 

 briefly summarized outline. 



