ANTISEPTIC AND GERMICIDAL POWER 151 



periods of time the garnets are taken out, the disinfectant 

 carried over removed by gentle washing, and, if necessary, by 

 washing in solvents (such as ammonium sulphide, if mercuric 

 chloride is used) to render inert any trace of disinfectant. 

 The bacteria or spores are then separated from the garnets by 

 shaking them in water. Definite amounts of the washings are 

 plated and the bacteria counted. 



The garnet method is a valuable one, but on the whole 

 one or other of the drop methods is the most convenient for 

 determining the germicidal action of any given substance for 

 the ordinary bacteria. The following are all of this character. 



Rideal- Walker method. 



It is convenient to compare the germicidal power with that 

 of some standard disinfectant. Rideal and Walker in 1903 1 

 introduced a method by which comparisons with carbolic acid 

 are made, under definite conditions. Their procedure gives a 

 good idea of the principles of the drop method. 



In the Rideal-Walker method a carefully standardized pure 

 carbolic acid solution is used as a control, accurate dilutions in 

 sterile distilled water being prepared. The carbolic acid solution 

 can be made up from the pure crystals, but they must be dry or 

 as dry as possible. The solution should be standardized by 

 means of titration with bromide. Dilutions must be freshly 

 made each day. A twenty-four hours' broth culture (Lemco), 

 grown at 37 C, of the B. typhosus or other organism tested, is 

 used. When B. typhosus is employed a reaction of + 1*5 for the 

 broth is recommended. 



The dilutions of the disinfectant are, in general, made with 

 distilled water. In this method they are usually placed in test 

 tubes which for convenience are frequently fitted in special 

 racks. The dilutions must of course be very carefully prepared, 

 and to avoid errors in diluting at least 5 c.c. of each strength 

 should be taken to make the dilution required. All diluting 

 solutions, instruments, flasks and test tubes must be sterilized 

 before use. 



1 Journal of Sanitary Institute, 1903, vol. xxiv, p. 424. 



