84 Plants and their Ways in South Africa 



(O), and a very small part of carbon-dioxide, "03 to 100. Carbon 

 dioxide is a gas compounded of one part of carbon and two of 

 oxygen, which hold on to each other very closely. CO^ is the 

 symbol or short form of writing this gas. Traces of other gases 

 are found. Hydrogen gas (H) is found free in extremely minute 

 quantities. It is combined though with O to form water 

 vapour. The proportion of nitrogen and oxygen in the air can 

 be shown by the following experiment : — 



Ex. 30. Invert a glass jar with straight sides over a pan of water. Notice 

 how high the water stands in the jar. Now lift the jar and place under it 

 apiece of phosphorus fastened to a cork to keep it afloat. (Phosphorus 

 should always be kept and cut under water.) The phosphorus burns, that 

 is, it unites with the oxygen, forming dense white fumes. After a time the 

 fumes disappear. Where have they gone ? The water has absorbed them 

 and formed phosphoric acid. What else has happened ? The water has 

 risen and fills about one-fifth of the jar. The acid occupies less space than 

 the gas fumes, so the weight of the air outside the jar forces the water up 

 to take the place of the oxygen. The nitrogen does not burn but fills 

 the remainder of the jar. 



Ex. 31. The amount of oxygen may be more accurately determined 

 by placing the phosphorus in a long specimen tube. The cubic contents 

 may be measured or the outside may be marked off into equal parts. 

 With a lens focus the sun's rays on to the phosphorus, or the tube may 

 be held in hot water until the phosphorus is melted and run down the 

 sides. The entire amount of oxygen will combine with the phosphorus. 

 After cooling, uncork the tube under water, mouth downward, and measure 

 the height of water which ascends in the tube. 



Water also absorbs CO2 and holds it in solution or forms 

 carbonic acid ; but there is a very small amount of carbonic 

 acid gas in the air. 



It is oxygen that we need for respiring. Nitrogen dilutes 

 the oxygen enough for us and for land-plants to breathe. But 

 fish and water-plants require the oxygen diluted still more with 

 water. If oxygen were not diluted with nitrogen we should 

 feel as uncomfortable " as a fish out of water ". 



Respiration is essentially the same process in plants as in 

 animals. Oxygen is inhaled which enters into chemical com- 

 bination with the carbon of the complex contents of the cell, 

 breaking them down and forming simpler substances. As a 

 result of this alteration carbonic acid gas and water are ex- 



