DISINFECTANTS AND ANTISEPTICS. 195 



Hill's Method. — The test-organism is dried upon the end 

 of a sterile glass rod contained in a sterile test-tube, the end 

 of the rod projecting through a cotton plug. The end of 

 a glass rod is immersed in a fluid culture of the test-organism 

 and allowed to dry. While drying it is inserted into a sterile 

 test-tube, and plugged around with cotton. It is then ready to 

 test by exposure to any germicide, either liquid or gaseous. 

 After exposure to the germicide it is plunged into a tube of 

 sterile beef-broth in order to see whether the organisms adher- 

 ing are all killed. 



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

 particularly in the testing of powerful germicides. In Stern- 

 berg'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 ger- 

 micide. This does not remove another source of error, namely, 

 the chemical action that may take place between the substance 

 and the protoplasmic contents of the bacterial cell. This action 

 may extend 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, the bacteria after 

 exposure to the germicide be inoculated into susceptible ani- 

 mals; but Sternberg's experiments in this direction have 

 shown that bacteria may become so attenuated in virulence 

 by the action of germicides insufficient to kill that the value of 

 animal inoculation experiments is limited. Moreover, it some- 

 times happens that it is desired to test germicides on bacteria 

 which are not pathogenic for animals. 



Geppert suggested a valuable modification of these methods 

 while determining the germicidal value of bichloride of mer- 



