NON-LIVING THINGS 211 



weight thus always borne, but they need it to keep them in 

 compact, solid form. Thus air seems to us as nothing, 

 except in its unusual manifestations. We commonly say 

 that a vessel is empty when it is full of air, and the fact 

 that air and other invisible gases are really forms of matter 

 is not always easy to make plain to the child mind. 



In the lower grades this fact should be made plain by 

 discussion and by simple experiments. Discuss with the 

 children the movements of air that can be felt, the blow- 

 ing of the wind ; rising currents from heated stoves ; move- 

 ments of clouds, which are simply masses of fog high above 

 the earth, floating in variable currents of air ; the movement 

 i dust and other light objects or even of small pebbles by 

 wind. We can hear, feel, and in a sense even see air. 

 supports the bird in its flight, the boys' kites, and toy 

 alloons. Under compression it is even used for cushions 

 nd mattresses; it can be made solid enough to sustain 

 reat weight, as when it is pumped into bicycle and auto- 

 lobile tires, thus being greatly compressed. 

 Invert a jar and force it mouth down into a vessel that 

 ontains water. Does the water rise to fill the jar? Why 

 ot? Fill a drinking glass nearly full of water. Put a 

 ard over its mouth and invert the glass, holding the card 

 uring the process. Now take the hand away from the 

 ard; it will be found that the card remains in place and 

 lat the water does not flow out. Why not? Now lower 

 ie glass mouth down into a vessel of water, and as soon as 

 is touching the water remove the card. The water still 

 emains in the glass. Why? With a crooked tube let air 

 ito the upper part of the glass, and the water flows out 

 ntil it reaches the level of that in the vessel. Why ? Fas- 



