644 A MANUAL OF BACTERIOLOGY 



(3) Rideal-Walker or drop-method. Moor first suggested that the 

 germicidal efficiency of a disinfectant might be compared with that 

 of a standard solution of carbolic acid, which has a definite com- 

 position, is stable, and can be accurately standardised, and Rideal 

 and Walker devised an ingenious and simple method for carrying 

 this out. A special test-tube rack is very convenient (Fig. 69), in 

 .which the lower tier has five holes which hold three or four tubes 

 containing the solutions of decreasing strengths of the disinfectant 

 tojbe tested, and two tubes or one tube containing standard carbolic 



FIG. 69. Test-tube rack with test-tubes arranged for the 

 Rideal-Walker method of testing disinfectants. 



acid solution of known strength for comparison. The upper tier 

 has thirty holes in two rows spaced into six sets of five holes 

 each. These hold tubes of sterile nutrient broth which are num- 

 bered from 1 to 30. The test is usually made with a broth culture 

 of B. typhosus, but other organisms may be employed. The process 

 is as follows : The five tubes in the lower tier each contain 3 c.c. 

 of the disinfectant and carbolic solutions. Into each in succession, 

 at intervals of half a minute, three drops of the typhoid broth 

 culture are added with a pipette. Half a minute after the last tube 

 has been inseminated, a loopful is taken from the first tube and 

 inseminated into the first broth tube, and this process is repeated 

 at half-minute intervals until all the broth tubes have been inocu- 

 lated. The inoculated broth tubes are then incubated at 37 C. 

 for three days, and the occurrence or not of growth is taken as 

 indicating the killing or non-killing of the organism respectively. 

 Obviously the first set of five broth tubes inoculated are subcultures 



