338 



GERMICIDAL ACTIVITY OF EUCALYPTUS OILS, 11., 



had the same resistance as the stock culture. In Table x., test 

 b. was made with the stock culture and test c. with race 6. 



The stock culture clearly contained a mixture of races, the 

 most resistant of which have played their part in the recorded 

 experiments. The germicidal dilutions are those which destroy 

 all the bacteria in certain times, and naturally the most resistant 

 bacteria will be the last to succumb. 



b. The Effect of Mass Infection — The lethal dilutions for the 

 stock culture in the previous test with the races of B. coli com- 

 munis are not in agreement with the results tabulated elsewhere. 

 This is explained by the fact that the infected broth added to 

 the 2 c.c. of disinfectant weighed 7 milligrams, while elsewhere 

 the infecting material weighed 25 milligrams (the droplet from 

 a standard pipette). The proof of this is found in the following: 

 Table xviii. B. to/i communis. 



Exposure in minutes ... 15 



E. australiana, 2nd hour. 



Infection, 25 mgrms. 1 : 400 

 7 nigrins. 1 : 600 



240 



800 

 1000 



It confirms the well known rule of mass infection, i.e., the 

 greater the number of bacteria, the stronger must be the disin- 

 fectant to destroy them. It emphasises the importance of keep- 

 ing to one method of technique in doing a set of experiments. 



c. The Effect of Acidity. — In a preliminary test, it was found 

 that when the oil of E. cinerea was diluted with water contain- 

 ing 1° of alkalinity as sodium bicarbonate, it was less germicidal 

 than when neutral distilled water was employed. It is possible 

 that the converse of this holds, and that slightly acid water 

 would increase the germicidal power. There is also the possi- 

 bility that an acid oil may be more toxic on account of the 

 acidity conferred upon the diluting water by the acid of the oil. 

 The elucidation of these two points was attempted. 



The chief and probably the most active free acid in the oils is 

 acetic, and accordingly this acid was tested to see if it had any 

 bactericidal power of its own. A solution was prepared by 



