196 MANUAL OF BACTERIOLOGY. 



the rod is inoculated with a small amount of a fluid culture 

 of the test organism and allowed to dry. It is then ready 

 to test by exposure to any disinfectant, either liquid or 

 gaseous. 



All of these methods are open to serious sources of error, 

 particularly in the testing of powerful germicides. In 

 Sternberg's method, small quantities of the substances 

 tested may be carried over with the organisms, and, if a 

 powerful germicide, in sufficient amount to prevent growth, 

 and thus give erroneous results. In Koch's or Hill's 

 method this factor may be partially obviated by washing in 

 sterile water after exposure to the germicide. This does not 

 remove another source of error, namely, the chemical action 

 that may take place between the substance and the proto- 

 plasmic contents of the bacterial cell. This action may ex- 

 tend deeply enough to restrain the growth of an organism 

 for a very long time without actually killing it. When 

 placed under suitable conditions, such union may be broken 

 up and the organism regain its power to develop. It has 

 been suggested that, to remove errors in the above methods, 

 test-cultures containing bacteria supposed to be killed by 

 the smallest quantity of germicide be inoculated into sus- 

 ceptible animals; but Sternberg's experiments in this direc- 

 tion have shown that bacteria may become so altered in 

 virulence by the action of germicides insufficient to kill, that 

 animal inoculation experiments are worthless. 



Geppert suggested a valuable modification of these 

 methods while determining the germicidal value of bichlo- 

 ride of mercury. After exposing his test-organism to 

 bichloride of mercury, and before inoculating into bouillon 

 to determine death of the organism, he treated with a dilute 

 ammonium sulphide solution, thus effectually neutralizing 

 any mercury-salt remaining. 



Sedgwick developed this method still further, and to him 



