226 DISCOURSE ON THE STUDY 



or less flexible and yielding. We know for certain, 

 that the space which any material body appears to 

 occupy is not entirely filled by it ; because there is 

 none which by the application of a sufficient force 

 may not be compressed or forced into a smaller space, 

 and which, either wholly, as in air or liquids, or in 

 part, as in the greater number of solids, will not re- 

 cover its former dimensions when the force is taken 

 off. In the case of air, this condensation may be 

 urged to almost any extent ; and not only does a 

 mass of air so condensed completely recover its ori- 

 ginal bulk, when the applied pressure is removed, 

 but if that ordinary pressure under which it exists 

 at the earth's surface (and which arises from the 

 weight of the atmosphere) be also removed by an air- 

 pump, it will still further dilate itself without limit 

 so far as we have yet been able to try it. Hence 

 we are led to the conclusion that the particles of air 

 are mutually elastic, and have a tendency to recede 

 from one another, which can only be counteracted by 

 force, and therefore is itself a force of the repulsive 

 kind. Nevertheless, as air is heavy, and as gravi- 

 tation is a universal property of matter, there is no 

 doubt that this repulsive tendency must have a 

 limit, and that there is a distance to which, if the 

 particles of the air could be removed from each 

 other, their mutual repulsion would cease, and an 

 attraction take its place. This limit is probably 

 attained at some very great height above the earth's 

 surface, beyond which, of course, its atmosphere 

 cannot extend. 



(240.) What, however, we can only conclude by 

 this or similar reasoning respecting air, we see dis- 



