2O Nature- Study Agriculture 



When they burn the change is more rapid, but here 

 again the carbon forms carbon dioxid, and goes into 

 the air with the smoke. When plants are eaten, the 

 animals that eat them breathe out the carbon dioxid. 



Experiments and Observations 



1. Hold a cold flatiron in a flame. The moisture (a com- 

 pound) that condenses on it is made by the uniting of hydrogen 

 from the fuel with oxygen from the air. 



2. With a knife or file scratch through the rust on a piece 

 of old iron. Why is the rust only on the surface ? 



3. Into nine small bottles (Fig. 12) put materials to repre- 

 sent the ten elements that are essential plant foods, as follows: 

 (i) Wood ashes (containing potassium), (2) lime or plaster (con- 

 taining calcium), (3) iron, (4) a little flash-light powder (magne- 

 sium), (5) saltpeter or air (containing nitrogen), (6) matches (a 

 compound of phosphorus), (7) sulfur, (8) charcoal (carbon), 

 (9) water (hydrogen and oxygen). 



4. Put a piece of perfectly dry wood into a test tube and heat 

 it over an alcohol flame. The water seen collecting in the tube 

 is made by the breaking down, in the wood, of compounds that 

 contain hydrogen and oxygen. 



5. Fill a glass with lime water and let it stand exposed to the 

 air for several days. The crust that forms on the surface of the 

 water results from the C0 2 gas of the air uniting with the lime in 

 the water, making a thin layer of limestone. 



6. That air contains water vapor is shown by the fact that 

 moisture from the air will condense on a pitcher of ice water. 



7. Into a Mason fruit jar put about two teaspoonfuls of 

 sodium peroxid (from a drug store). Pour water upon it and 

 cover the jar loosely. The jar will soon fill with oxygen. If a 

 stick with a glowing ember at the end is inserted in the jar, the 

 ember will immediately burst into flame. 



8. To show that seeds will not grow without air, put a few 

 wheat grains into a large bottle and half fill a similar bottle with 



