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LECTURE XXII. 



ON PNEUMATIC EQUILIBRIUM. 



J. HE laws of the pressure and equilibrium of liquids, which are the peculiar 

 subjects of hydrostatics, are also appHcable in general to fluids of all kinds, 

 as far as they are compatible with the compressibility of those fluids, or with 

 their tendency to expand. 



Elastic fluids are distinguished from liquids by the absence of all cohesive 

 force, or by their immediate tendency to expand when they are at liberty. 

 Such are atmospheric air, steam, and gases of various kinds; and the consi- 

 deration of these fluids, in the state of rest, constitutes the doctrine of pneu- 

 niatostatics, or of the equilibrium of elastic fluids. 



That the air is a material substance, capable of resisting pressure, is easily 

 shown, by inverting an empty jar in water ; and by the operation of transfer- 

 ring airs and gases from vessel to vessel, in the pneumatic apparatus_xused by 



' chemists. The tendency of the air to expand is shown by the experiment in 

 which a flaccid bladder becomes distended, and shrivelled fruit recovers its 



^ full size, as soon as the external pressure is removed from it, by the operation 

 of the air pump: and the magnitude of this expansive force is more distinctly 

 seen, when a portion of air is inclosed in a glass vessel, together with some 

 mercury, in which the mouth of a tube is immersed, while the other end is 

 open, and without the vessel; so that when the whole apparatus is inclosed in 

 a very long jar, and the air of the jar is exhausted, the column of mercury 

 becomes the measure of the expansive force of the aif. (Plate XIX. Fig. 

 S51.) 



If the diameter of the tube, in an apparatus of this kind, were very small in 

 comparison with tlie bulk of the air confined, the column of mercury would 



