SUPERIOR LETHAL COEFFICIENT 313 



the agar culture, using a loopful of disinfectant solu- 

 tion of the different percentages to prepare the emulsion 

 on each cover-slip. 



4. Examine microscopically and note the strongest 

 solution which does not cause plasmolysis and the 

 weakest solution which does plasmolyse the organism. 



5. Make control preparations of these two solutions 

 and determine the percentage to be tested. 



6. Pipette 10 c.c. sterile water into the culture bottle 

 and suspend the entire surface growth in it. 



7. Transfer the suspension to the Erlenmeyer flask 

 and mix it with the 90 c.c. of sterile water remaining 

 in the flask. 



8. Pipette 10 c.c. of the diluted suspension into each 

 of ten sterile test-tubes. 



9. Label one of the tubes "Control" and place it 

 in the incubator at 18 C. 



10. Add to each of the remaining tubes a sufficient 

 quantity r of a concentrated solution of the disinfectant 

 to produce the percentage previously determined upon 

 (vide step 5). 



11. Incubate the tubes at 18 C. to 20 C. 



12. At hourly intervals remove the control tube 

 and one of the tubes with added disinfectant from the 

 incubator. 



13. Make a subcultivation from both the control and 

 the test suspension, upon the surface of nutrient agar ; 

 incubate under optimum conditions. 



14. Observe these culture tubes from day to day 

 until the completion of seven days, and determine 

 the shortest exposure necessary to cause the death of 

 vegetative forms. 



Superior Lethal Coefficient. 



i. Prepare surface cultivations of the "test" organ- 

 isms upon nutrient agar in a culture bottle, and incu- 



1 See Percentage Formula, Appendix, page 496. 



