TENSION OF GASES. 163 



8. A certain room is 10 m. long, 8 m. wide and 4 m. high, (a.) 

 What weight of air does it contain ? (&.) What is the pressure upon 

 its floor? (c.) Upon its ceiling? (d.) Upon each end? (e.) Upon 

 each side? (/.) What is the total pressure upon the six surfaces? 

 (g.) Why is not the room torn to pieces ? 



9. An empty toy balloon weighs 5 g. When filled with 10 I. of 

 hydrogen, what load can it lift ? (See Appendix, G.) 



Recapitulation. In this section we have considered 

 the definitions of Pneumatics and Tension ; the 

 Aerial Ocean in which we live ; the mechanical 

 Properties of Air ; the weight of air giving rise to 

 Atmospheric Pressure; a famous experiment by 

 Torricelli,and the explanation thereof;' Pascal's ex- 

 periments and the conclusion they confirmed ; the Ba- 

 rometer ; the Aneroid barometer ; the Baro- 

 scope. 



ECTfON II. 



THE RELATION OF TENSION AND VOLUME TO 

 PRESSURE. 



282. Tension of Gases. If a glass flask, provided 

 with a stop-cock, be closed under an atmospheric pressure 

 which supports a mercury column of 30 inches, the atmos- 

 pheric pressure from without is exactly balanced by the 

 tension ( 269) of the air within. If it be closed under a 

 barometric pressure of 28 inches, this equality of the two 

 pressures will continue. If the flask be closed when the 

 surrounding air is subjected to a pressure of two or three 

 atmospheres, the equality will still continue. In none of 

 these cases will the glass be subjected to any strain because 



