PNEUMATICS. 165 



are particularly distinguished from liquids, which resist compression, and pos- 

 sess but a small degree of elasticity. Illustrations of the compressibility of air 

 are most familiar. A quantity of air contained in a bladder, or India-rubber 

 bag, may be easily forced by the pressure of the hand, to occupy less space. 

 There is, indeed, no theoretical limit to the compression of air, for with every 

 additional degree of force, an additional degree of compression may be obtained. 



^ . The elasticitv, or expansibility of air, also manifests itself 



Does air pos- . . > r- j \ 



Bess any con- in an unlimited degree. Air cannot be said to have any 



volume ?^^ °^ original size or volume, for it always strives to occupy a 



larger space. 



What are illus- When a part of the air inclosed in any vessel is withdrawn, 



trations of the ' 



expansibility that which remains, expanding by its elastic property, always 



of air ? gjls the dimensions of the vessel as completely as before. If 



nine tenths were withdrawn, the remaining one tenth would occupy the same 

 space that the whole did formerly. 



This tendency of air to occupy a larger space, or in other words, to increase 

 its volume, causes it when confined in a vessel, to continually press against 

 the inner surfltce. If no corresponding pressure acts from the outer surface, 

 the air will burst it, unless the vessel is of considerable strength. This fact may 

 be shown by the experiment of placing a bladder partially filled with air be- 

 neath the receiver of an air-pump, and by exhausting the air in the receiver 

 the pressure of the external air upon the outer surface of the bladder is re- 

 moved. The elasticity of the air contained in the bladder being then unre- 

 sisted by any external pressure, will dilate the bladder to its fiillest extent, 

 and oftentimes burst it. 



v, . . ^ . 369. Air, as well as all other g-ases and va- 



.HasaiT weight? ' , ° 



pors, possesses weight. 



The weight of air may be shown by first weighing a suitable vessel fiUed 

 with air ; then exhausting the air from it by means of an air-pump, and weigh- 

 ing again. The difference between the two weights will be the weight of the 

 air contained in the vessel. 



The weight of 100 cubic inches of air is about 31 grains. 



To what is the ^'^^^ "^^^ elasticity, or expansion of air is due 

 ^rlulT °^ **^ ^^^ peculiar action of the molecular forces 

 among its particles, which manifest themselves 

 in a very different manner from what they do in solid and 

 liquid bodies. 



In solid bodies, these forces hold the molecules, or particles together so 

 closel3', that they can not change their respective positions; they also hold 

 together the particles of liquid bodies, but to such a limited extent only, as to 

 enable the particles to move upon each other with perfect freedom. But in 

 gases, or aeriform substances, the molecular forces act repulsively, and give to the 

 particles a tendency to move away from each other; and this to so great an ex- 

 tent, that nothing but external impediments can hinder their further expansion. 



