BACTERIA AND PHYSICAL ENVIRONMENT 17 



2. When the agar is soHdified, invert the Petri dish and cover 

 half the bottom with black paper. 



3. Expose this dish, paper side up, to direct sunlight for half 

 a day, or, better still, place it on a table and adjust a 40-candle- 

 power incandescent electric Ught about 50 cm. above it while the 

 colonies are developing. 



4. Record the differences in the number and size of the col- 

 onies on both the illuminated and the darkened side of the dish. 



Exercise 18. Effect of Light of DiSetent Wave Lengths upon 

 Bacterial Growth 



To test the effect of primary colors upon bacterial growth : 

 1—2. Same as in the preceding exercise. 



3. Coat the exposed half of the dish with photographic collo- 

 dion which contains 2 per cent of one of the following aniline 

 dyes : Chrysoidine (for red) ; Aurantia (for orange) ; Naples yel- 

 low (for yellow) ; Malachite green (for green) ; Eosin, bluish 

 (for blue) ; Methyl violet (for violet). Different students may 

 employ different dyes. 



4. Expose the plates thus prepared to strong diffused light 

 (not direct sunlight) for several days. 



5. Take notes upon the growth of colonies on both sides of 

 the dish. Observe the results where lights of different colors are 

 used. 



Exercise 19. Relation of Bacteria to Heat 



Make four agar streak cultures of B. coli and four of B. mega- 

 therium. Incubate one of each at the following temperatures : 

 7° C. (ice chest), 20° C. (room), 28° C. (incubator), 37.5° C. 

 (incubator). 



Keep careful notes on the amount and rate of growth of cul- 

 tures, and determine the optimum temperature of these organisms. 



Exercise 20. Relation of Bacteria to Concentration of Medium 



Place about 3 cc. of Liebig's Extract of Beef in each of two 

 large test tubes. Add to one tube 20 cc. of water. Inoculate 

 both with B. fluorescens Kquefaciens. In which tube do bacteria 

 develop ? Why ? 



