242 PATHOGENIC BACTERIA. 



planted. It is always well to use a considerable number 

 of tubes. Bouillon is generally employed. If the anti- 

 septic is non-volatile, it may be added before sterilization, 

 which is to be preferred; but if it is volatile, it must be 

 added by means of a sterile pipette, with the greatest 

 precaution as regards asepsis, immediately before the test 

 is to be made. Control-experiments — i. e. without the 

 addition of the antiseptic — should always be made. 



The results of antiseptic action are two: retardation of 

 growth and complete inhibition of growth. As the tubes 

 used for the study of the antiseptic are watched in their 

 development, it will usually be noticed that those con- 

 taining very small quantities develop almost as rapidly as 

 the control-tubes; those containing more, a little more 

 slowly; those containing still more, very slowly, until at 

 last there comes at time when the growth is not deferred, 

 but prevented. 



Sternberg points out that certain circumstances may 

 modify the results obtained. They are: 



i. The composition of the nutrient media, with which 

 the antiseptic may be incompatible. 



2. The nature of the test-organism, no two organisms 

 being exactly alike in their susceptibility. 



3. The temperature at which the experiment is con- 

 ducted, a relatively greater amount of the antiseptic 

 being necessary at temperatures favorable to the organ- 

 ism than at temperatures unfavorable. 



4. The presence of spores which are always more 

 resistant than the asporogenous forms. 



II. The Germicidal Value. — Koch's original method of 

 doing this was to dry the micro-organisms upon sterile 

 shreds of linen or silk, and then soak them for varying 

 lengths of time in the germicidal solution. After the 

 bath in the reagent the threads were washed in clean, 

 sterile water and then transferred to fresh culture- 

 media, and their growth or failure to grow observed. It 

 will be observed that this method is aimed at the deter- 

 mination of the time in which a certain solution will kill. 



