308 METHODS OF IDENTIFICATION AND STUDY 



7. At intervals of five hours open the apparatus, 

 remove one of the cover-slip films from the Petri 

 dish, and transfer it to the interior of a culture flask, 

 with every precaution against contamination. Re- 

 seal the desiccator and again exhaust, and subsequently 

 admit dry sterile air as before. 



8. Incubate the culture flask under optimum condi- 

 tions until the completion of seven days, if necessary; 

 and determine the time exposure at which death occurs. 



9. Pour plates from those culture flasks which grow, 

 to determine the absence of contamination. 



10. Repeat these observations at hourly intervals for 

 the five hours preceding and succeeding the death 

 time, as determined in the first set of experiments. 



(B) Light. 



(a) Diffuse Daylight : 



i. Prepare a tube cultivation in nutrient bouillon, 

 and incubate under optimum conditions, for forty-eight 

 hours. 



FIG. 159. Plate with star for testing effect of light. 



2. Pour twenty plate cultivations, ten of nutrient 

 gelatine and ten of nutrient agar, each containing o.i 

 c.c. of the bouillon culture. 



3. Place one agar plate and one gelatine plate into 

 the hot and cold incubators, respectively, as controls. 



4. Fasten a piece of black paper, cut the shape of a 

 cross or star, on the centre of the cover of each of the 

 remaining plates (Fig. 159). 



