RIDEAL-WALKER METHOD 647 



or faeces reduced the carbolic acid coefficient of some proprietary 

 disinfectants to a greater relative extent than that of carbolic. 



The method is also sometimes somewhat erratic in practice, and 

 a number of determinations may be needed before the strengths of 

 disinfectant and carbolic which coincide are found. Occasionally 

 also two strains of B. typhosus may differ widely as regards the 

 germicidal action of the disinfectant on them, while they are prac- 

 tically identical as regards the germicidal action of the carbolic. 



Woodhead and Ponder have proposed a modification of the 

 method. In this, B. coli is used as the test-organism and bile-salt 

 peptone water as the culture medium, a platinum spoon being used 

 for culturing, and more cultures at shorter intervals up to half 

 an hour are made. 



4. Volatile disinfectants may be tested by moistening the wool 

 plug of an agar tube, inoculating the agar, and capping with a 

 rubber cap, and observing whether any growth occurs. 



5. Volatile disinfectants may also be tested by exposing silk 

 threads, pieces of paper or fabrics, splinters of wood, etc., impreg- 

 nated with organisms, some free, others done up in packets of cotton- 

 wool, in a room or chamber of known cubic capacity, to the action 

 of the gas, a known amount of which is present in the chamber. 

 After exposure for a given time, the threads are sown in broth 

 tubes, and the tubes incubated. 



On the Rideal- Walker method, etc., see Rideal and Walker, 

 Journ. Sanitary Inst., vol. xxiv, 1903, p. 424 ; Kenwood and 

 Hewlett, ibid. vol. xxvii, 1906, p. 1 ; Firth and Macfadyen, ibid. 

 p. 17 ; Kenwood, Public Health, 1908 ; Fowler, Journ. Roy. Army 

 Med. Corps, July 1907 ; Partridge, Bacteriological Examination 

 of Disinfectants ; Woodhead and Ponder, Lancet, 1909, vol. ii. 



