DISINFECTION, STERILIZATION AND ANTISEPSIS. 243 



1. To a measured quantity of a virulent bouillon-culture 

 of the test-organism is added a given amount of the germi- 

 cide. After varying lengths of time, inoculations are made from 

 this mixture into culture-media, preferably bouillon, and note 

 is made of the presence or absence of growth under suitable 

 conditions of temperature and the like. The shortest exposure 

 to the weakest solution of the substance necessary to kill the 

 test-organism is taken as the germicidal value of that substance 

 for the particular organism used. 



2. Small pieces of sterile silk or cotton thread are soaked 

 for some hours in a bouillon-culture of the test-organisms. 

 The threads are then removed, partially dried and placed in a 

 solution of known strength of the germicide and exposed for a 

 definite length of time. The thread is removed from the solu- 

 tion, washed carefully in sterile water, dropped into a tube of 

 sterile bouillon plugged with cotton, and growth or absence 

 of growth noted. As in other methods, the greatest dilution 

 of the germicide that will kill the test-organism in the shortest 

 ti-me is taken as the germicidal value of that substance for the 

 organism used. Spores of the hay bacillus may be used when 

 experiments are being made by large classes of students. 



3. 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 rea,dy 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 adhering are all 

 killed. 



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

 ticularly in the testing of powerful germicides. In method No. i, 

 small quantities of the substances tested maybe carried over with 



