RIDEAL-WALKER METHOD 



775 



for carrying this out. A special test tube rack is very con- 

 venient (Fig. 68), in which the lower tier has five holes which 

 hold three or four tubes containing the solutions of decreasing 

 strengths of the disinfectant to be tested, and two tubes or one 

 tube containing standard carbolic 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 numbered 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 



* 't^ 1 



FIG. 68. Test-tube rack with test-tubes arranged for the Rideal- 

 Walker method of testing disinfectants. 



five tubes in the lower tier each contain 5 c.c. of the disinfectant 

 and carbolic solutions. Into each in succession, at intervals of 

 half a minute, 0-2 c.c. of the typhoid broth culture is 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 inoculated. 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. It will 

 be seen that the first set of five broth tubes inoculated are sub- 

 cultures in which the organism has been acted upon by the 

 disinfectant and carbolic solutions for two and a half minutes, 



