PRESSURE OF AIR AND LIQUIDS 173 



The children will readily see that the pressure of a liquid is 

 due to its weight, and consequently is greater in the heavier 

 liquids. It will be very interesting to show a very light liquid, 

 like gasoline (do not use it near a light), and the heavy mercury. 



From liquids it is easy to pass to air. In previous lessons 

 air has been shown to be " something," and consequently has 

 weight and exerts pressure. Let the children devise means of 

 showing this to be true. 



Put experiments like this in the form of questions: A glass 

 of water evenly full is covered with a piece of paper, and then 

 suddenly inverted, the paper being held on by the palm of the 

 hand. "When the hand is removed, the water will be kept in the 

 glass by the pressure of the air. 



" Sucking" water up a tube is removing part of the pressure of 

 the air above the water in the tube. The pressure of the air on 

 the water outside of the tube pushes the water up the tube. 



Enlarging the chest in respiration nmkes a larger space. 

 The pressure of the outside air crowds in and inflates the lungs 

 to fill that space. 



The toy called a "sucker" — a leather disc with a string in 

 the center — illustrates this further; the pump and the siphon also. 



From pressure of air, we may pass to experiments on 

 bodies floating in the air, as balloons, — toy balloons, either paper 

 filled with hot air or rubber filled with a gas lighter than air. 

 Next we may take up currents in the air and water. 



It has been seen that heat expands water and air. These are 

 then lighter. The warm water rises in the cold, and the warm 

 air in the cooler. (The lessons on currents in air and water may 

 now be referred to or they may be arranged to follow these 

 lessons.) 



