30 THE AIR. 



together with about one part in every thousand of carbonic 

 acid (consisting of carbon and oxygen in union) and a small 

 proportion of vapour of carbonate of ammonia (ammonia in 

 union with carbonic acid). It is the oxygen which enables 

 animals to live in the air, as it is taken into their systems at 

 every breath, it is thus constantly being consumed and 

 would ultimately be so far diminished, that they could live 

 no longer for want of it ; but Providence has so arranged, 

 that the necessary quantity ik always being supplied by 

 vegetables (which give out oxygen) to meet the deficiency. 

 The air, although a gas, is yet capable of being weighed, of 

 great compressibility, and of expansion to an unlimited 

 extent, this causes the lower part (near the surface of the 

 earth) to be much more dense than the upper regions, and 

 those who have ascended to the tops of high mountains, have 

 described the difficulty of respiration as being very great, 

 owing to the rarefaction or lightness of the air. At the 

 surface of the earth it presses with a weight of about fifteen 

 pounds upon every square inch, but as this pressure is equal 

 in every direction, it is not felt by us, nor does it crush the 

 most fragile flower or insect ; but, remove the pressure from 

 one side of anything, and it will be found to prefs with 

 violence upon the other. If the air were removed from the 

 inside of a drum, the weight of the surrounding air would 

 burst in the parchment and fill it. By way of experiment, 

 fill a large basin with water, take a tumbler in the left 

 hand and a piece of lighted paper in the right, hold the 

 lighted paper for a moment under the inverted tumbler and 

 immediately apply its open mouth to 

 the surface of the water, letting it dip 

 in about half an inch, the heat expands 

 and consequently gets rid of some of the 

 air in the tumbler, and as this cools 

 again it resumes its original bulk ; the 

 pressure of the air on the surface of the 

 PIG water in the basin will force it up into 



the tumbler and nearly fill it (fig. 1). 

 This is the principle of the barometer, which is a tube entirely 

 exhausted of air and the weight of the atmosphere forces a 



