RIDEAL-WALKER METHOD 145 
controls of a suitable strength are also prepared. Five c.c. of each 
of these dilutions are placed in sterile test tubes, to which are added 
at intervals of one-half minute a twenty-four-hour broth culture of 
B. typhosus in the proportion of 1 drop of culture to each cubic centi- 
meter of disinfectant used (according to Partridge, 1 drop of culture 
equals about 0.1 c.c.). 
At the end of two and a half minutes a loopful of each of the mix- 
tures is inoculated into a test tube containing 5 c.c. of standard broth, 
an interval of half a minute being thus allowed between taking the 
samples from the different dilutions. This is repeated at five, seven and 
a half, ten, twelve and a half, and fifteen minutes. The broth tubes, 
after being incubated at 37° C. for forty-eight hours, are examined for 
growth. 
The results of the examination are then noted, and if suitable 
comparative strengths of the disinfectant and carbolic acid have been 
selected the carbolic acid coefficient is determined as above stated. 
The following table (Table 10) illustrates the manner of determining 
the carbolic acid coefficient of a disinfectant according to the Rideal- 
Walker method: 
TABLE X 
Name, “ A.” 
Temperature of medication, 20° C. 
Culture used, B typhosus, twenty-four-hour, extract broth, filtered. 
Proportion of culture and disinfectant, 01 ¢.c.+5 ce. 
Time Culture Exposed to Action 
of Disinfectant for Minutes 
Sample Dilution jo Phenol Coefi- 
cient. 
Phenol. ...... 0c. cece eee ees 1: 90 +)—-}]-]- 100)550 
1:100); +)/+)+])-j|]r-{- 5.5 
coefficient. 
Disinfectant “A”.......... 1:500; +i +i -i-}|-]- 
1:550;}/ +); +)+)-;-4j- 
1:600; +); 4+;+/}/+)]-[- 
This method has had to be greatly modified. There are certain 
things taken for granted which may materially alter the results. It is 
assumed that the rate of death is proportional to the concentration 
of the disinfectant. Bacteria are regarded as molecules in their reaction 
with the disinfectant. Chick has recommended thirty minutes as the 
standard time in place of fifteen minutes. She pointed out the error 
