144 STERILIZATION AND DISINFECTION 
in disinfection it is the aim to destroy bacteria and the reaction must 
take place between the bacterial ccll and the disinfectant. 
The bactericidal activity of chemicals has been studied by many 
investigators. Robert Koch in the early days of the science gave 
some attention to the question. He devised a method wherein the 
bacteria were dried on silk threads. These were then suspended in 
known dilutions of the disinfectant. After the process had been com- 
pleted these threads were then transferred to media in order to deter- 
mine whether the bacteria had been killed. He used Bacillus anthracis 
for the test organism. Sternberg mixed 5 c.c. of a young culture 
with equal quantities of the germicide. At regular intervals, loops of 
this mixture were removed and putin broth. Kronig and Paul (1897) 
dipped garnets into a culture of Bacillus anthracis containing spores 
and allowed them to dry. These garnets were then put into the dis- 
infecting solutions and a few taken out at intervals. They were 
shaken in a definite amount of water and plates made from this. In 
this way they determined the rate of death. They realized that the 
value of any disinfectant varied with the conditions under which 
it acted. 
Upon such data as these our present methods have been devised. 
These represent the present stage in the effort to secure satisfactory 
methods with which to measure strength of disinfectants. Walters 
(1917) has pointed out that disinfectants possess a certain specificity 
for certain bacteria. He further argues that the strength of a disin- 
fectant may only be stated in terms of the organism on which it is to 
be used in actual practice. 
Rideal-Walker Method. Anderson and McClintic (1912) have 
summed up this method as follows: 
Briefly stated, the carbolic coefficient in the Rideal-Walker method 
is arrived at by dividing the figure indicating the degree of dilution 
of the disinfectant that kills an organism in a given time by that 
expressing the degree of dilution of the carbolic acid that kills the 
same organism in the same time under exactly similar conditions. 
Leaving out details, the determination of the Rideal-Walker coefficient 
is substantially as follows: 
Certain standard conditions are considered essential to the proper 
performance of the test. Phenol solutions of known strength are 
used; cultures are grown in a standard medium, transplants being 
made every twenty-four hours; the loops used for all inoculations are 
of a standard size (about 4 mm. in diameter). Usually four dilutions 
of suitable strengths of the disinfectant to be used are made. Phenol 
