16 ELEMENTARY BIOLOGY. [ll- 



[B. Physiology. 



Get some water that is qnite green from containing a 

 large quantity of Protococcus ; introduce some of it 

 into two tubes inverted over mercury, and pass a small 

 quantity of carbonic anhydride into each : keep one tube 

 in the dark and place the other in bright sunlight for 

 some hours. Then measure the gas in each tube and after- 

 wards introduce a fragment of caustic potash into each ; 

 the gas from the specimen kept in the dark will be more or 

 less completely absorbed ( = carbonic anhydride), that from 

 the other will not be absorbed by the potash alone, but will 

 be absorbed on the further introduction of a few drops of 

 solution of jjyrogallic acid (= oxygen). Protococcus, there- 

 fore, in the sunlight, takes up carbonic anhydride and 

 evolves oxygen. A comparative experiment may be made 

 with a third tube containing water but no Protococcus.] 



