RIDEAL-WALKER METHOD 



645 



in which the organism has been acted upon by the disinfectant and 

 carbolic solutions for two and a half minutes, the second set for 

 five minutes, and so on. The results (taken from an actual test) 

 may be charted as follows : 



B. typhosus, 24:-hour broth culture at 37 C. 

 Room -temperature 60 F. 



= growth in the sub-cultures. 



= no growth in the sub-cultures. 



From this it will be seen that the disinfectant X in a solution of 

 1 in 1600 kills in the same time (7| minutes) as carbolic 1 in 100. 

 This result is expressed as a coefficient obtained by dividing the 

 strength of disinfectant by the strength of carbolic which kills each 

 in the same time ; in the present instance the co-efficient is Y Q = 

 16-0, and this figure is known as the " carbolic acid coefficient." 



If nothing is known about the strength of the disinfectant, some 

 preliminary experiments should be performed with dilutions at 

 wide intervals as regards strength (e.g. 1-100, 1-500, 1-1000, 

 1-1500, 1-2000, etc.), and when the limit has thus been approxi- 

 mately ascertained, the test is performed as above. 



Precautions to be taken in carrying out the test. (1) The culture 

 should be a broth one about twenty to twenty-four hours old, and 

 should be free from clumps ; this may be attained by filtration 

 through paper. Instead of adding drops of the culture to the 

 solutions, the addition of 0-1 c.c. of culture for every cubic centi- 

 metre of solution has recently been suggested. The writer regards 

 this amount as being too large, and would suggest that O'l c.c. of 

 culture is sufficient. 



(2) The carbolic acid (the crystals of which should have a melting- 

 point of not less than 40-5 C.) should be kept in the form of a 

 5 per cent, aqueous solution standardised by the bromine method. 



