14 



PNEUMATICS. 



in contact with the bottom ; 

 and thus placed, let it be se- 

 cured at E, so as not to permit 

 the escape of the air which 

 fills the part of the bottle 

 above the surface of the mer- 

 cury, and which is thus of the 

 same density and temperature 

 as the atmosphere. Let the 

 whole apparatus thus arranged 

 be introduced under the re- 

 ceiver of an air-pump, and let 

 the air be gradually withdrawn. 

 The pressure of the air on the 

 surface of the mercury in the 

 tube C D being gradually di- 

 minished, the elasticity of the 

 air in AE, pressing on the 

 surface F, will force the mer- 

 cury to ascend in the tube ; and 

 the height of the mercury thus 

 pressed into the tube, will in- 

 crease as the pressure of the 

 air in the receiver is diminished 

 by rarefaction. The column 

 of mercury suspended in the tube, at 

 any period of the process, will evidently 

 indicate the excess of the elasticity of 

 the air in EF, or of the external air 

 above the elasticity of the rarefied air in 

 the receiver. 



Now suppose another tube GH (fig* 

 14.), open at both ends, 

 and exactly similar to the 

 former, be made to com- 

 municate at one end G 

 with the receiver R ; and 

 let the other end H be 

 plunged in - a cistern of 

 mercury, open at the top, 

 and exposed to the atmos- 

 pheric pressure. As the 

 rarefaction proceeds, the 

 pressure of the air in the 

 tube G H being gradually 

 diminished, the weight of 

 the atmosphere pressing 

 on the surface I, will force 

 the mercury up in the 

 tube G H ; and the alti- 

 tude of the column, at 

 any stage of the process, 

 will indicate the excess 

 of the weight of the at- 

 mosphere above the elas- 

 ' tic force of the rarefied 

 air in the receiver. 



Now, upon comparing 

 the altitudes of the co- 

 lumns in the two tubes 

 GH and CD, in every 

 stage of the process it will be found that 



fig. 14. 



J 



the columns of mercury suspended in 

 them, above the levels of the mercury 

 in their respective cisterns, are accu- 

 rately equal. From whence it follows, 

 that the weight of the atmosphere press- 

 ing on the surface L, and its elastic force 

 pressing on the surface F, exceed the 

 elastic force of the rarefied air in the 

 receiver by the same quantity ; and, 

 therefore, that the weight or pressure of 

 the atmosphere is exactly equal to its 

 elasticity. 



(31.) There are many familiar ef- 

 fects, which are only consequences of 

 the elasticity of air. Beer or ale, bottled, 

 contains in it a quantity of air, the elas- 

 tic force of which is resisted by the 

 pressure of the condensed air between 

 the cork and the surface of the liquid in 

 the bottle. On removing the cork the 

 liquid, and the air which it contains, 

 are relieved from this intense pressure. 

 The liquid itself not being elastic, is not 

 affected by this ; but the elastic force of 

 the condensed air which has been fixed 

 in it, having no adequate resistance, it 

 immediately escapes, and rises in bubbles 

 to the surface, and produces the frothy 

 appearance consequent upon opening 

 the bottle. 



If a shrivelled apple be placed under 

 the receiver of an air-pump, and the air 

 be withdrawn, it will have its coat dis- 

 tended by the internal air, so as to pre- 

 sent a perfectly smooth appearance, 

 Also, if a thin glass bottle, with atmos- 

 pheric air confined in it, be placed under 

 a receiver, it will burst by the elasticity 

 of the enclosed air when a sufficient ex- 

 haustion has been produced. 



CHAPTER V. 

 On the Air-Pump. Condenser. 



I. The Air-Pump. 



(32.) In philosophical investigations 

 it frequently becomes necessary that the 

 substances which are the subjects of 

 experiment should be removed from the 

 influence, whether mechanical or che- 

 mical, of the atmosphere. For this 

 purpose it is desirable, that we should 

 possess the means of withdrawing the 

 air from a glass vessel called a receiver, 

 in which the substance is placed, and 

 through which the changes which it 

 suffers may be observed. The space 

 under the glass vessel after the air has 

 been withdrawn from it is called a va^ 

 cuum, and the machine by which the 

 air is withdrawn is called an air-pump. 



