34 THE BIOLOGY OF THE CELL 



they will be exposed to the sunlight. Examine 

 the tubes from time to time for two or three days. 

 In which tube does the yeast grow better ? Has 

 sunlight any effect upon the growth of yeast ? 



d. Nature of the gas given off hy the growing yeast. 



1. Take two test-tubes, a perforated cork which will 



fit one of the test-tubes, and a U-shaped glass 

 tube, one branch of which is passed through 

 the perforation in the cork. Fill the first test- 

 tube one-third full of clear baryta water, or 

 lime-water may be used if necessary ; into the 

 second test-tube put some baking-soda or bicar- 

 bonate of soda (which is a combination of so- 

 dium and carbon dioxide, or " carbonic- acid 

 gas ") ; pour into the second test - tube a few 

 drops of dilute muriatic or of dilute sulphuric 

 acid ; insert the cork at once in such a manner 

 that the end of one arm of the U-shaped tube 

 is above the surface of the fluid in the test-tube ; 

 hold the first test-tube in such a manner that 

 the end of the other arm of the U-shaped tube 

 will extend ielow the surface of the baryta 

 water in this test-tube. The gas {oarhon diox- 

 ide) formed in the second test-tube by the 

 action of the acid upon the soda will thus be 

 carried by the U-shaped tube over into the first 

 test-tube, where it will bubble up through the 

 baryta water. What change takes place in the 

 baryta water ? 



2. Pour some clear baryta water into a watch-glass, 



and, holding the mouth close to the surface of 

 the liquid, breathe heavily through the mouth 

 upon the surface of the water. What change 



