LESSONS IN PHYSICS. 39 



be illustrated the compressibility and expansibility of air. Blow 

 gently into the end of the delivery tube and the air will be com- 

 pressed ; apply the hand to the test tube and the air will expand 

 as shown by the movement of the fluid in the delivery tube. 

 The elasticity of the air may also be shown when illustrating 

 its compressibility. Experiments with other gases show that 

 they possess these properties in common with air. 



Gases press in all directions. Fill a common tumbler with 

 water and cover it with a piece of paper a little larger than the 

 top of the glass; then, holding the paper in place with the fingers, 

 invert the glass, and the pressure of the air on the paper will 

 hold the water in the glass. It will do the same when the glass 

 is turned sidewiser This experiment shows that air presses up- 

 ward and side wise, and surely it must press downward, must 

 have weight. The specific gravity of gases is usually found by 

 comparing them with the weight of an equal bulk of hydrogen 

 or air. Objects floating in the air, as leaves, feathers, birds, bal- 

 loons, etc., show that air is buoyant, behaving much as liquids 

 do, and the destructive force of winds indicates that it must 

 have weight and other properties like liquids and solids. 



Invert a tumbler under water, as in the experiment for im- 

 penetrability, and raising it partly out of the water, notice that 

 the water is higher inside the glass than on the outside. The air 

 can not press on the water inside the tumbler, but the pressure of 

 the air on the water outside keeps the water up on the inside. 

 This experiment shows the downward pressure of the air and 

 illustrates the principle of the common pump and the barometer. 



Take a glass tube a little more than thirty inches long, closed 

 at one end, and fill it with mercury; then, closing the open end 

 with the finger, invert the tube, and place the end closed by the 

 finger in a cup of mercury; then withdraw the finger. The press- 

 ure of the air on the surface of the mercury in the cup will sus- 

 tain a column of mercury nearly thirty inches high in the tube. 

 Experiments of this kind show that the pressure of the air at the 

 level of the sea is about 15 pounds to the square inch, sustaining 

 a column of mercury about 30 inches high, or a column of water 



