THE CHICK-MARTIN TEST 147 



The Chick-Martin Test. — In the Rideal- Walker method the disinfectant acts 

 in pure solution. But, in practice, disinfectants have usually to act in solutions 

 containing organic matter. As the presence of organic matter seriously lessens 

 the activity of most disinfectants. Chick and Martin (1908) suggested that the 

 disinfectant should be tested on the organisms not in distilled water, but in water 

 containing a suspension of 3 per cent, dried human faeces. Further, instead of 

 allowing the time to vary, as in the Rideal- Walker method, they fix a time limit 

 of 30 minutes for the action of the disinfectant, making subcultures at the end 

 of this time. The phenol coefficient of disinfectants, especially those of the emul- 

 sified disinfectants, is distinctly lower by this method. Thus whereas the activity 

 of phenol was reduced about 10 per cent, in the presence of 3 per cent, faeces, that 

 of the commercial cresols was reduced 30-50 per cent. 



There are certain general principles governing the estimation of disinfectant 

 power that must be rigidly adhered to. Great care must be taken to use media 

 that are alike in composition and in H-ion concentration. The organisms used 

 for the test must always be grown and subcultured under exactly the same con- 

 ditions. Disinfectants such as HgClj, which, as we have already seen, are antiseptic 

 even in extremely high dilution, must be neutralized, preferably with a saturated 

 solution of HjS, at the termination of their action, and thioglycollate must be added 

 to the medium in which subcultures are made to provide the necessary — SH 

 groups. With phenol, dilution is unnecessary, as the inoculation of a looj^ful of 

 suspension into 5 ml. of broth dilutes the phenol sufficiently to destroy its antiseptic 

 action. In comparing salts, molar concentrations, either in their multiples or sub- 

 multiples, should be used. We have seen that the great majority of organisms are 

 killed long before the culture is completely sterilized ; there are always a few 

 organisms that resist disinfection for a considerable period after the remainder are 

 destroyed. Therefore any method that takes sterilization as its end-point is bound 

 to be fallacious. This is one of the greatest objections to all the current methods. 

 Similarly any method in which the time is fixed is unsatisfactory. 



Even when the best available standard methods are employed, and the tem- 

 perature is kept constant, there still remain very important theoretical objections. 

 The present methods depend on the fixation of all but one variable, and the com- 

 parison of difierent disinfectants on this variable. Thus, under standard con- 

 ditions we learn the concentrations necessary to kill a given organism in a given 

 length of time. It is obvious that the results we obtain must vary with the con- 

 ditions we lay down. Suppose the time taken be 2-5 minutes, as it sometimes is 

 in the Rideal-Walker method, then for the sterilization of Sahn. parafi/phi the 

 phenol coefficient of HgClg would work out at just over 2 ; but if the time taken is 

 30 minutes, as it is in the Chick-Martin test, the phenol coefficient works out 

 at 550 (Chick 1908). Again, an increase in the concentration of phenol increases 

 the rate of disinfection far more than a similar increase in the concentration of 

 HgClj ; doubUng the concentration of the former shortens the reaction 64 times, 

 of the latter only twice. The effect of temperature also is important, for a rise in 

 temjjerature increases the rate of action with some disinfectants, such as phenol 

 and the emulsified disinfectants, far more than with others, such as the salts of the 

 heavy metals. 



The Rideal-Walker method has been severely and justly criticized. It tells us 

 nothing about the effect of change in concentration or in temperature ; it measures 

 only the reaction velocity at a given concentration and during a particular interval 



