64 Plants and their Ways in South Africa 



is given off when Plants breathe. — Make some baryta-water 

 by dissolving barium carbonate in water and allow it to settle. 

 Pour some of the clear water into a test-tube ; breathe into it 

 through a glass tube. The water becomes cloudy and a white 

 film collects on the surface.^ Place some baryta- water in a 

 shallow dish. Uncover the jar of peas and pour the gas into the 

 dish. We have seen it is heavier than air, and so can be poured. 

 A white film appears on the surface. The peas have exhaled 

 the same gas which you breathed into the jar, carbon dioxide. 



Another pretty experiment shows that carbon dioxide is 

 given off by plants while breathing. Place heads of flowers — 

 daisies just opening are good ones — in a flask. Invert the 

 flask in soda-limewater, which absorbs carbon dioxide. 

 Pour some mercury into the soda-limewater. In a short time 

 the water and then the mercury will be drawn up into the neck 

 of the flask as the carbon dioxide is absorbed. If you do not 

 see why, recall that when a gas is reduced into a liquid it 

 occupies less space. The weight of the air outside the flask 

 pressed on the mercury until it fills up the space occupied by 

 the carbon dioxide, which is now absorbed into the soda-lime- 

 water. It will continue to rise until the gas inside the flask 

 presses down on the mercury as much as the air outside is 

 pressing it up. 



Place the flask in the dark and see if the mercury continues 

 to rise. Mark the height at which the mercury stands at night, 

 and again in the morning. A change in temperature aflects 

 the height. When the flask cools, the gas within occupies less 

 space and the mercury rises. 



Plants get energy to do work by breathing. Unless 

 our rooms are supplied with plenty of fresh air we become dull 

 and unable to work. Breathing decreases our weight but sets 

 energy free to do work. Ask some one who plays football 

 how much weight the team loses while playing a cup match. 

 The stroke oar in a rowing match will lose six or seven pounds 

 in twenty minutes or during a four-mile race. How hard you 

 breathe when you are playing. Did you ever try to get warm 



^ Liniewater may l)e used by dissolving calcium carbonate in water. 

 The results are not quite so good. These liquids should be kept corked. 



