OBJECTS.] INTRODUCTORY. 53 



downwards into a glass jar full of water. If the flask 

 were empty there would be no reason why the water 

 should not enter the neck of the flask and stand at 

 the same height inside the neck as it does outside. 

 If you take an " empty " glass tube open at each end 

 and press it down into the water, the water inside and 

 the water outside will stand at the same level. But if 

 you put your finger on the upper end of the tube so 

 as to convert it into a closed vessel, the water will 

 enter the lower end only a little way. So with the 

 flask, the water enters the neck only a little way. 

 Hence there is something inside the "empty" tube 

 and in the "empty" flask; something which is mate- 

 rial, because it occupies space and offers resistance. 

 In fact the flask is full of that form of matter which 

 is termed air, a thick coat of which surrounds the 

 earth as the atmosphere. Air has weight, as you 

 will learn more fully by and by; and that air in 

 motion can transfer that motion to other bodies you 

 are taught by the effects of the winds, which are 

 merely air in motion. 



Air therefore has all the characters of a material 

 substance. Moreover it is a fluid, for it fits itself 

 exactly to the shape of any vessel which contains it ; 

 its parts are very easily moved, or we should feel its 

 resistance every time we move a limb ; that it "flows " 

 is seen in every breeze and every time you use a pair 

 of bellows, when the air is driven in a stream out of 

 the nozzle; and it presses on all sides anything 

 contained in it. 



But though air is a fluid it is not a liquid. In the 

 first place it is very compressible. We saw that the 

 water entered a little way into the tube or the neck cf 



