146 



DISINFECTION 



f erred to a solid medium in order to ascertain whether all the spores had been killed. 

 This method is open to certain fallacies to which attention was first drawn by 

 Geppert (1889, 1891a, b). Geppert found that if, after a short time in mercuric 

 chloride solution, the spores were transferred to a culture medium, they failed 

 to grow, but that if they were inoculated into a guinea-pig, they gave rise to 

 anthrax. The reason for this appeared to be that sufficient HgClg was carried over 

 by the thread into the broth to prevent germination of the spores occurring ; 

 in the tissues of the guinea-pig, however, this quantity was neutralized, and the 

 spores that were still alive were able to develop. The truth of this explanation 

 was shown by the fact that cultivation of the spores proved successful, provided 

 the threads were treated with ammonium sulphide to neutralize the mercury 

 before being inoculated into the broth. The thread method, therefore, yielded 

 higher values than the disinfectant actually possessed. Geppert's work was of 

 considerable importance, particularly in relation to disinfection by salts of the 

 heavy metals. These substances have a low concentration exponent, and there- 

 fore act as antiseptics even in high dilution. Since it is impossible by this method 

 to remove all traces of the disinfectant from the interstices of the thread, Kronig 

 and Paul (1897) replaced the threads by garnets ; they introduced a further 

 improvement, which rendered the test quantitative, by plating out the washings 

 from the garnets, and counting the number of colonies that developed. The 

 two tests that are chiefly used at the present time, either in their original or in a 

 modified form, are the Rideal-Walker and the Chick-Martin methods. 



The Rideal-Walker Drop Method (Rideal and Walker 1903). — In this method 

 similar quantities of organisms are submitted to the action of varying concentra- 

 tions of phenol and of the germicide to be tested. Subcultures are made into 

 broth every 2| minutes up to 15 minutes and the tubes incubated at 37° C. 

 for 3 days. That dilution of disinfectant X which sterilizes the suspension in 

 a given time is divided by that dilution of phenol which sterilizes the suspension 

 in the same time, and a phenol coefficiept obtained. Thus : 



TABLE 20 

 Salm. typhi, 24 HotiRS' Broth Ctjlture at 37° C. Temperature at which Test was 



CONDUCTED 60° F. 



grow . ^^^ phenol coefficient of X is therefore -— - = 2-1. 



— = no growth. ^ 130 



(For a full discussion of the Rideal-Walker method see Lancet, 1909, ii, 1516.) 



