PNEUMATICS. 



13 



(-27.) Throughout all this process, 

 however, we have supposed that the 

 temperature of the air remains the same. 

 For if the temperature be elevated, an 

 increase of elasticity will ensue, without 

 any change in the density ; nay, even 

 with a diminution of the density. For 

 if a flaccid bladder be secured at the 

 mouth so as to stop the transmission of 

 air, and be then heated, the air within it 

 will expand by its increased elastic 

 force, and at length the bladder will be 



Eerfectly filled, and have a tendency to 

 urst. In this instance, th'e increased 

 space occupied by the air in the bladder, 

 proves that its density is diminished ; 

 notwithstanding which, its elastic force 

 is increased. 



(28.) Liquids transmit pressure only 

 when some mechanical force produces 

 a corresponding pressure on some part 

 of their surface. Elastic fluids, how- 

 ever, ' exert a pressure quite indepen- 

 dently of this. To explain this more 

 fully,' let A B (Jig. 12.) be an horizontal 

 tube, filled with a liquid, and closed at 

 both ends. If we set aside the tendency 

 of the fluid to fall out at the ends by its 

 weight, the ends sustain no pressure 

 from it. If any pressure be exerted at 



fig. 12 



one end A by a piston, the other end B 

 will sustain an equal pressure ; but upon 

 removing the pressure at A, the other 

 end B will be immediately relieved from 

 it. Now, if, instead of a liquid, we 

 suppose the tube filled with air, the case 

 will be quite otherwise ; for each end 

 will then sustain a pressure outwards, 

 by reason of the elasticity of the air, and 

 which will be equal exactly to the weight 

 of the atmosphere, if the air in the tube 

 have the same density and temperature 

 as the external air. That there is this 

 outward pressure must be evident, when 

 we consider, that if there were not, the 

 pressure of the atmosphere would force 

 the ends AB inwards, with a force 

 equal to its weight. We can, however, 

 give more direct proofs of this pressure 

 of air against the inner surface of any 

 vessel in" which it is contained, and show 

 the means whereby the amount of this 

 pressure may be ascertained. 



(29.) Let a flaccid bladder, containing 

 a small quantity of air, be placed under 

 an inverted glass receiver, and let the 

 air be withdrawn from the receiver by 



means of the air-pump. As the air is 

 gradually withdrawn, the bladder will 

 apparently become inflated, and will at 

 length assume the same appearance as 

 if a quantity of air had been forced into 

 it. But no air has been introduced into 

 it, nor does it, when thus apparently in- 

 flated, contain more air than it did when 

 in its flaccid state. What then, it will be 

 asked, produces the apparent inflation ? 

 On the removal of the air from the 

 receiver by means of the pump, its 

 pressure on the external surface of the 

 bladder is removed ; the elastic force of 

 the air contained in the flaccid bladder 

 is then no longer opposed by the resist- 

 ing pressure of the external air, and it 

 accordingly takes effect ; and its pres- 

 sure on the inner surface of the bladder 

 swells it in the same manner as if, 

 while the pressure of the external air 

 remained, the pressure of the confined 

 air had been increased by the introduc- 

 tion of an additional quantity. In this 

 case, the inflation or swelling of the 

 bladder is produced by a very different 

 cause from that which produced it when 

 heated. (27.) In that case, while the 

 pressure of the atmosphere on the ex- 

 ternal surface remained the same, the 

 elastic force pressing on the interior sur- 

 face was increased by heat. On the 

 other hand, in the present instance, the 

 elastic force of the air contained in the 

 bladder is Jess after it has swelled, and 

 apparently filled, than it was when 

 flaccid. For the air it contains conti- 

 nues of the same temperature, while the 

 space it occupies is increased. Its 

 density, and therefore its elastic force 

 (26.), are thus diminished. The cause 

 of apparent inflation is the diminution 

 of the external pressure of the atmos- 

 phere by the pump ; so that although 

 the elastic force of the air in the blad- 

 der be diminished, yet it is not so much 

 diminished as the pressure of the exter- 

 nal air, and so predominates and swells 

 the bladder. 



(30.) This experiment merely esta- 

 blishes the fact, that air without con- 

 densation does press on the inner surface 

 of the vessel which encloses it. We 

 shall now, however, describe a method 

 of measuring this pressure : 



Let AB (Jig. 13.) i be a cylindrical 

 glass bottle, containing a quantity of 

 mercury to the level F. Let a glass 

 tube C D, open at both ends, and up- 

 wards of thirty-two inches in length, be 

 introduced at the neck E of the bottle, 

 so that the lower end D shall be nearly 



