VAPOR. 77 



Give the exam- 180. Thus, as much water will rise in 

 P les - vapor into a jar of air as if it were a va- 



cuum. And, in addition to this, as much alcohol and 

 ether successively, as if the jar were entirely empty. 

 The tension or pressure from within, outward, is, of 

 course, increased by each additional vapor. 



181. MOIST AIR LIGHTER THAN DRY. 



Why is moist 



air lighter It would naturally be supposed that air 

 than dry air ? conta j n j n g moisture, would be heavier than 

 air containing none. And it would be so, but for the 

 fact that the presence of vapor causes the air to ex- 

 pand slightly, and grow lighter, and this to an extent 

 more than sufficient to compensate for the increase of 

 weight. 



BOILING. 



182. WEIGHT OF THE ATMOSPHERE. 



lt As an introduction to the subject of boil- 



atmosphere ing^ jt will be necessary to consider the 



has weight ? - 



pressure of the atmosphere. The earth is 

 surrounded by an atmosphere, estimated to be fifty miles 

 high. It is very light compared with the earth itself, or 

 with water. But it has weight, as may be proved by 

 weighing a bottle full of air, and then pumping out 

 the air and weighing it again. The empty bottle will 

 be found to weigh less than the bottle full of air. 



183. ANOTHER PROOF OF THE WEIGHT 



Give another 



proof that air OF THE AIR. That the air has weight, is 

 has weight. a g a i n proved by tying a piece of bladder 



