ELEMENTS 



193 



ring stand, so that the other end of the tube shall dip below the 

 surface of water in a large vessel (see Fig. 130). Into the tube 

 put 5 grams of potassium chlorate (KC1O 3 ) mixed with 5 grams 

 of manganese dioxide (MnO 2 ). Stop the tube tightly and heat it. 

 Bubbles of gas soon appear in the water. 

 Now fill two or three pint jars with water; 

 tip one bottom upward under water and 

 hold it over the tube so that the gas shall 

 go up into it. Be sure that the jar is full 

 of water at the start, and allow no air to 

 enter it. As the gas flows, the water in the 

 jar is pushed down and out. When the jar 

 is full of gas (i.e. the water is all out of it), 

 cover it with a piece of stiff cardboard or 

 glass and lift it from the water. In the same way fill two other 

 jars, and keep each covered (Fig. 131). The gas is O. By heat- 

 ing, KC1O 3 is decomposed into KC1 and 3 O. 



Experiment 120. Into one jar of O put a glowing splinter of 

 wood. In another hold a bit of burning sulphur (Fig. 132). In 

 the third place a lighted bit of candle. Be 

 careful to keep the jars covered as much of the 

 time as possible. In each case what do you 

 notice when the burning substance is first put 

 into the jar ? What do you notice after it has 

 burned a few moments ? Try to explain this. 



FIG. 131 



FIG. 132 



These substances seem to burn better 

 in the jar of pure O than in the air. In 

 either case the burning is a process of 

 chemical combination, the substance com- 

 bining with oxygen ; in the jar the O is 

 nearly pure, while in the air it is mixed with a much 

 larger quantity of another gas (N), so that the bodies 

 burn better in the jar. Note carefully that when sub- 

 stances burn in air (e.g. wood, kerosene, paper, etc.), it 



