DISINFECTION AND DISINFECTANTS 259 



medication, the age of the culture, the species, and even 

 the strain, of the organism, the number of organisms pre- 

 sented for disinfection, the reaction of the culture medium, 

 and the influence of organic matter, must all be con- 

 sidered. The desirability of stating the values of 

 disinfectants in terms of a common standard is obvious, 

 as it allows comparison, and the standard invariably 

 used is absolute phenol (first suggested by C. G. Moor). 

 The examination of a disinfectant involves, first, the 

 determination of its value in comparison with that of 

 phenol when the organisms are presented ' naked ' - 

 i.e., without any appreciable amount of extraneous organic 

 matter present; and, secondly, when such organic matter 

 as is likely to be met with in practice is introduced into 

 the test to ascertain what, if any, is the depreciation in 

 value caused thereby. One method only of testing a dis- 

 infectant against a naked organism (the Rideal- Walker 

 method) need be considered in any detail, as this particular 

 method has been universally adopted for the purpose. 

 It requires a certain amount of dexterity and much care 

 to perform, but it is remarkable for the manner in which 

 factors likely to cause discrepancies are recognised and 

 guarded against. 



Rideal -Walker Method. A special test-tube rack is 

 used: a lower tier contains five holes for four tubes, hold- 

 ing the strengths of the disinfectant to be tested, and one 

 tube containing the standard phenol. The upper tier has 

 spaces for thirty test-tubes, in six sets of five holes. In 

 the upper tiers are placed tubes of standard broth, and 

 the tubes are numbered 1 to 30. 



The method may now be briefly described as follows: 

 To 3 c.c. of a particular dilution of the disinfectant in 

 sterile water add 3 drops of a twenty-four hours' blood- 

 heat culture of the organism in broth; shake, and take 

 subcultures every two and a half minutes up to fifteen 

 minutes. Incubate these subcultures for at least forty- 

 eight hours at 37 C. Allowing thirty seconds for each 

 act of medication and the same time for making each 

 subculture, four different dilutions of the disinfectant under 

 examination, together with one standard control, may be 

 tested against the same culture, under conditions which 

 make the results strictly comparable. No table is to be 

 considered complete which does not show a positive result 



