DISINFECTION AND DISINFECTANTS 263 



disinfectant solution is made up and allowed to stand for 

 several days before it is completely used, so if the dilution 

 necessary is calculated upon a freshly made emulsion, 

 which afterwards separates, and which in this way has a 

 lower efficiency than that calculated, it is quite conceiv- 

 able that considerable damage might be done. Rideal 

 and Walker, therefore, suggest that before applying the 

 test to a disinfectant a 1 per cent, solution should be 



Srepared at normal temperature in a stoppered litre 

 ask, the portion required for further dilutions in the 

 test to be pipetted from the top of the flask after twenty- 

 four hours' subsidence, avoiding the withdrawal of any 

 insoluble matter which may be floating on the surface. 

 The coefficient is then referred to as a ' permanent 

 coefficient.' 



The Lancet Commission (see Lancet, November 13, 20, 

 and 27, and December 18, 1909; Pharm. Journ., July 30, 

 1910) used a considerably modified Rideal-Walker test. 

 B. coli was used as the test organism, MacConkey's bile- 

 salt broth for the secondary subcultures, and platinum 

 cups were substituted for the inoculating needle. The 

 test covers thirty minutes as against the fifteen minutes 

 for the Rideal-Walker test, and more dilutions both of 

 the disinfectant and phenol are used. In determining 

 the coefficient, the weakest dilutions of disinfectant and 

 phenol killing in two and a half minutes are compared, 

 and a coefficient for the thirty-minutes period is obtained 

 in the same way. The mean of these two figures is 

 taken. This method is no advance on the Rideal-Walker. 

 When bacteria have been exposed to non lethal dilutions 

 of disinfectant, they are ' sick.' To put these into 

 MacConkey media, which cuts out weak organisms, 

 introduces an indecisive factor into the comparison. 

 Anderson and McClintic, of the United States Public 

 Health Service, advocate keeping the typhoid bacillus 

 as the test organism, as there is greater variability 

 between strains of colon bacilli than between typhoid 

 strains. 



Anderson and McClintic (Journ. Infect. Dis., viii. 1), in 

 their ' Hygienic Laboratory Method,' compare an average 

 of two and a half and fifteen minute periods, and use an 

 increased number of dilutions of both disinfectant and 

 phenol. So much work has to be done in the time that 



