Gravitation 17 



had pumped the air out, it took sixteen horses, eight 

 pulling one way and eight the opposite way, to pull the 

 hemispheres apart. 



Experiment 7. Fill a bottle (or flask) half full of water. 

 Through a one-hole stopper that will fit the bottle, put a bent 

 piece of glass tubing that will reach down to the bottom of 

 the bottle. Set the bottle, thus stoppered, on the plate of 

 the air pump, with a beaker or tumbler under the outer end 

 of the glass tube. Put the bell jar over the bottle and glass, 

 and pump the air out of the jar. What is it that forces the 

 water up and out of the bottle? Why could it do this when 

 the air was pumped out of the bell jar and not before? 



How a seltzer siphon works. A seltzer siphon works 

 on the same principle. But instead of the ordinary com- 

 pressed air that is all around us, there is in the seltzer 

 siphon a gas (carbon dioxid) which has been much more 

 compressed than ordinary air. This strongly compressed 

 gas forces the seltzer water out into the less compressed 

 air, exactly as the compressed air in the upper part of 

 the bottle forced the water out into the comparative 

 vacuum of true bell jar in Experiment 7. 



Experiment 8. Fill a toy balloon partly full of air by 

 blowing into it, and close the neck with a rubber band so 

 that no air can escape. Lay a saucer over the hole in the 

 plate of the air pump, so that the rubber of the balloon 

 cannot be sucked down the hole. Lay the balloon on top 

 of this saucer, put the bell jar over it, and pump the air 

 out of the jar. What makes the balloon expand? What is 

 in it? Why could it not expand before you pumped the 

 air out from around it ? 



A toy balloon expands for the same reason when it 

 goes high in the air. Up there the air pressure is not 



