270 



Common Science 



FIG. 149. Filling a test tube with gas. 



the free end of a rubber tube that is attached to a gas pipe 

 and put it into the test tube a short distance, so that the 

 gas will go up into the tube, as shown in Figure 149. Now 

 turn on -the gas gently. When all the water has been forced 

 out of the tube and the gas bubbles begin to come up on 

 the outside, turn off the gas. Put a piece of cardboard, about 

 an inch or so square, over the mouth of the tube so that no 

 air can get into it, and take the tube out of the water, keeping 

 the mouth down and covered. Bring the empty test tube, 

 which of course is full of air, mouth up under the test tube 

 full of gas, making the mouths of the two tubes meet with 

 the cardboard between them, as shown in Figure 150. Now 

 have some one pull the cardboard gently from between the 

 two test tubes, so that the mouths of the tubes will be pressed 

 against each other and so that practically no gas will escape. 

 Hold them quietly this way, the tube of gas uppermost, for 

 not less than one full minute by the clock. A minute and a 



