266 NATURE STUDY AND AGRICULTURE 



hydroxide. What happens to iron of any sort when left 

 exposed to the air? 



Another simple experiment to show a chemical com- 

 bination may be performed. Place a small piece of sulphur 

 on a bright silver coin and set the sulphur on fire. When 

 it has burned look at the coin. What has taken place? 

 The dark substance is silver sulphide. A chemical com- 

 bination has taken place between the sulphur and the 

 silver. Was all the sulphur used in making the silver 

 sulphide ? What did you see taking place above the coin ? 

 Was anything uniting with the sulphur to form the blue 

 flame or smoke? This compound is sulphur dioxide. It 

 is a gas, and escaped into the air. How did you know it 

 was in the air? 



Now in plants chemical combinations take place which 

 result in the plant products we have found. Starch is a 

 combination of oxygen, hydrogen, and carbon. The 

 symbols for these are O, H, and C respectively. Proteid 

 is a combination of carbon, hydrogen, and oxygen with 

 nitrogen, sulphur, phosphorus, and often other elements. 



Plants not only succeed in uniting elements into com- 

 pounds, but in some cases they separate a compound into 

 its elements and use the elements to make a new compound. 



We may succeed in separating some compounds into 

 their elements. 



Experiment. — Heat in a test tube a level teaspoonful of 

 potassium chlorate mixed with the same amount of man- 

 ganese dioxide. After a few minutes hold a lighted splinter 

 in the mouth of the tube. What happens? Is there 

 something in the tube that was not there at the beginning, 

 even though you cannot see it ? The heat has caused the 



