LESSON x TESTING OF DISINFECTANTS 127 



B. METHODS OF TESTING DISINFECTANTS. 



(i.) Koch's method (modified). 



(a) Test the following solutions : 



Absolute alcohol ; 

 Mercuric chloride, 1 : 1000 ; 

 Mercuric iodide, 1 : 2000 ; 

 Carbolic acid, 1 : 20 ; 

 Condy's fluid. 



(1) Keep anthrax silk threads 1 in these solutions for 

 equal periods of time, varying from two to twenty-four 

 hours. 



(2) Then take them out of the fluid with sterile forceps 

 or needles. 



(3) Wash the threads in sterile water ; and do the same 

 with a thread that has not been kept in a disinfecting 

 solution. 



(4) Now press the threads into the substance of an agar- 

 agar plate, which must be kept at 38*5 C. Small labels 

 must be fastened on the lid of the Petri's capsule, in order 

 to avoid confusion. 



Examine the plate from day to day. 



1 Anthrax silk threads are prepared by making, under strictly 

 aseptic precautions, a suspension in broth or *6 per cent saline solution 

 of virulent agar-agar or potato cultures of anthrax bacilli, grown at 

 38 '5 C. for several days, and known to consist almost entirely of 

 spores. 



Sterilised silk threads, about an inch in length, are allowed to soak 

 in this suspension for half an hour to one hour, are then collected in a 

 sterile capsule, and dried in the warm incubator. 



