34 AIR — COMBUSTION. 



out feeling any resistance, and, from its being invisible 

 and totally unlike anything else we know, many for- 

 get its existence altogether. The fact is, that every 

 part of the surface of the globe is surrounded by air, 

 which floats on its surface almost like water. 



22. It is easy to prove that the air is really a sub- 

 stance. When we try to squeeze together the sides 

 of an inflated bladder, the mouth of which is tightly 

 tied up, we feel that the bladder is full of something 

 which resists the pressure ; this something is the air 

 which it contains, and which, though so easily dis- 

 placed, or pushed aside, by anything moving through 

 it, resists strongly any force applied to it when thus 

 confined in a limited space ; and if whilst we are press- 

 ing the bladder we prick a hole in it, the air then 

 rushes out, we feel that the resistance is gone, and 

 the sides of the bladder are easily squeezed together. 



23. And again, when working a pair of bellows, it 

 is the resistance of the air which we have to overcome 

 by the force of the arms, which constitutes the labor 

 of working the bellows ; and if the nozzle of the bel- 

 lows is stopped up, we presently find that it is impos- 

 sible to go on working the bellows any longer, because 

 having forced in as much air as it can hold, the natu- 

 ral tendency of the air to resist compression prevents 

 any more from entering. 



24. Although we are so forgetful of the very exist- 

 ence of the air, it is of the greatest importance to all 

 our daily occupations, and even to life itself. With- 

 out air nothing could burn; we could have neither 



