296 Common Science 



is forced out; so you can tell how much gas has collected 

 at any time by seeing how much water is left in each tube. 



One tube should fill with gas twice as fast as the other. 

 The gas in this tube is hydrogen; there is twice as much 

 hydrogen as there is oxygen in water. The tube that fills 

 more slowly contains oxygen. 



When the faster-filling tube is full of hydrogen that is, 

 when all of the water has been forced out of it take the 

 electrode out and let it hang loose in the glass. Put a piece 

 of cardboard about i inch square over the mouth of the 

 test tube; take the test tube out of the water and turn it 

 right side up, keeping it covered with the cardboard. Light 

 a match, remove the cardboard cover, and hold the match 

 over the open test tube. Does the hydrogen in it burn? 



When the tube containing the oxygen is full, take it out, 

 covered, just as you did the hydrogen test tube. But in this 

 case make the end of a stick of charcoal glow, remove the 

 cardboard from the tube, and then plunge the glowing char- 

 coal into the test tube full of oxygen. 



Only oxygen will make charcoal burst into flame like 

 this. 



When people found that they could take water apart 

 in this way and turn it into hydrogen and oxygen, and 

 when they found that whenever they combined hydrogen 

 with oxygen they got water, they knew, of course, that 

 water was not an element. Maybe some day they will 

 find that some of the eighty-five or so substances that we 

 now consider elements can really be divided into two or 

 more elements ; but so far the elements we know show 

 no signs of being made of anything except themselves. 



The last section of this book will explain something 

 about the way the chemist goes to work to find out what 

 elements are hidden in compounds. 



