CH.VIII ATMOSPHERIC PRESSURE AND BOYLE'S LAW 107 



it. Then open the stop-cock ; air will be heard to rush into 

 the flask, and as it does so the balance will show an increase of 

 weight. 



If the air is completely removed from a flask by means of an 

 air-pump, the difference in the weighings, before and after, will 

 provide the exact weight of 

 a given volume of air. Thus 

 if the vessel has a capacity 

 of a cubic foot the difference 

 of weight will be found to 

 be nearly an ounce and a 

 quarter. 



Pressure of the Atmo- 

 sphere. It has been seen 

 that it is a property of all 

 fluids that they communicate 

 pressure in all directions, 

 and consequently it is a char- 

 acter of air. It is a con- 

 sequence of this fact that 

 AVC are able to move about 

 quite freely in spite of atmo- 

 spheric pressure. Our bodies 

 are subjected to an enor- 

 mous pressure due to the 

 whole weight of the atmo- 

 sphere above us, and yet we 

 are quite ignorant of it, at 

 all events under ordinary 

 circumstances. Why is this ? 

 The lungs which fill up a 

 large part of our chest 

 capacity are inflated with 

 air, and other inside parts of 



the body are similarly in free communication with the atmo- 

 sphere. The inside air is under just the same pressure as 

 that outside, and consequently there is an exact compensation, 

 and we are not crushed, as the student will perhaps have expected 

 we ought to be. 



Exi'T. 102. Procure a thin tin can having a neck, into 

 which fits an india-rubber stopper. Take out the stopper and 



FIG. 46. Experiment to show that Air has 

 Weight. 



