RIDEAL-WALKER METHOD 777 



of a 5 per cent, aqueous solution standardised by the bromine 

 method. At 17-18 C. the strength of the carbolic control should 

 lie between 1-90 and 1-110. 



(c) All measures, pipettes, and test-tubes used for making dilutions 

 should be sterile. 



(d) The dilutions of the disinfectant and carbolic should be 

 made with sterile distilled water. 



(e) The broth used for culturing and subculturing should have 

 the following composition : 



The medium should be standardised to a reaction of -f 10 

 (Eyre's scale). 



(/) The loop used for subculturing should have an internal 

 diameter of 3 mm., and be made with platinum wire of 27-28 

 B.W.G. (a 4 mm. loop is now recommended). 



(g) Growths in the subcultures should be obtained in those 

 taken at not less than two and preferably at three of the time inter- 

 vals (two and a half, five, and seven and a half minutes) from both 

 the disinfectant and the carbolic solutions which correspond. 



(h) The temperature at which the determination is made should 

 be noted, and the strength of carbolic varied accordingly (e.g. 

 1-100 for 56-62 F., 1-1 10 for 62-67 F., and 1-120 for 67-73 F. 

 for B. typhosus), or the determination may be made at a standard 

 temperature (e.g. 20 C.) by warming (or cooling) the disinfectant 

 and carbolic tubes in a water-bath. 



(i) When the organism does not form a uniform culture in 

 broth, a suspension of an agar or other culture must be made in 

 water and filtered. Subculturing in some cases (e.g. with B. 

 pestis and B. anthracis) must be made on agar or other suitable 

 culture medium. 



The method is an admirable one for determining the relative 

 efficiencies of disinfectants on naked organisms in the absence of 

 organic matter. But in practice disinfection is almost always 

 carried out in the presence of organic matter, and various sugges- 

 tions have been made with a view of introducing this factor 

 into the test, for the presence of organic matter may reduce the 



