62 



approximately equal parts and put the equal portions 

 into clean test tubes of the same shape and size. Set 

 one tube in a bright light and one in a dark place, but 

 keep both tubes at the same temperature. Determine 

 by observation whether light has any effect on the growth 

 of the yeast plant in the mixture, and write a short de- 

 scription of each tube at the expiration of ten or twenty- 

 four hours. 



If a dark place is not available, wrap the tube in heavy 

 black paper or cloth. 



Does sunlight aid or inhibit bacterial growth ? 



Write a short account of the relation of sunlight to 

 bacterial diseases. 



Name five bacteria which are harmful to man. 



Name five bacteria which are useful to man. 



EXPERIMENT 51. THE CHEMICAL ACTION OF LIGHT 



(a) Dissolve a crystal of potassium bromide in a test 

 tube about one third full of water and then add an equal 

 quantity of silver nitrate solution. On shaking the test 

 tube, a precipitate of silver bromide is formed. Describe 

 this precipitate in your notebook. 



(b) Filter this precipitate in the usual manner, and 

 then expose the moist filter paper to the sunlight. De- 

 scribe the change which occurs, and tell what caused the 

 change. How is this property of silver bromide utilized ? 



EXPERIMENT 52. ILLUMINATION AND COLOR 



Soak some sheets of asbestos for several hours in a 

 saturated salt solution. Remove the sheets and after allow- 



