CHAPTER VIII 



ATMOSPHERIC PRESSURE AND BOYLE'S LAW 



SURROUNDING the earth in every latitude, over land and sea, 

 is a gaseous envelope which is spoken of as the air or the atmos- 

 phere. Its presence when at rest is imperceived, though in 

 motion it becomes apparent, since, by imparting its velocity to 

 trees and other bodies free to move, it affords a demonstration 

 of its existence. The student has already learnt to regard it as 

 a form of matter, and as consequently possessing weight. The 

 following experiments supply sufficient evidence of its existence 

 and of its weight. 



EXPT. 99. Invert a so-called empty bottle under water in 

 the pneumatic trough. Notice the bubbles which rise as the 

 water flows into the bottle. The water displaces the air which 

 thus becomes apparent. 



EXPT. 100. Move quickly across the room with a drawing 

 board in your hands. First hold the board " end on " and then 

 "broadside on." Notice that in the first case little or no 

 resistance is felt, while in the second, one's motion across the 

 room is considerably impeded. 



Weight of the Air. It is easy to prove by direct experiment 

 that the air has weight. 



EXPT. 101. Fit a one-holed indiaj^ubber stopper into a 

 fairly large glass flask, and fit into the stopper a short tube with 

 a stop-cock upon it (Fig. 46). Put a little water in the flask ; 

 open the stop-cock ; and boil the water. After boiling for a 

 little time, turn off the tap and place the flask on one side 

 to cool. When the flask is cool, weigh it, or counterpoise 



