AIR-PUMP. 



73 



before alluded to are closed up by strong iron doors ; 

 a continued draught of air supplies the fire, and 

 creates a salutary circulation through any part of the 

 vessel into which the pipes may be directed. They 

 are made either of copper or lead. 



AIR-PUMP ; a machine for the purpose of with- 

 drawing the air from some vessel or cavity, and 

 thereby making what is called a vacuum. It is one 

 of the most curious and useful of philosophical in- 

 struments. By experiments with it, the weight, 

 elasticity, and many other properties of a-ir may be 

 shown in a very simple and satisfactory manner. 

 Let R be the section of a glass bell, closed at the 

 top T, but open at the bottom, and liaving its lower 

 edge ground smooth, so as to rest in close contact 

 -with a smooth brass plate, of which SS is a section. 

 This glass is called a receiver, because it receives 

 and holds substances on which experiments are to be 

 made. If a little unctuous matter be rubbed upon 

 the edge of the receiver R, and it be pressed with a 

 slight circular motion upon the plate SS, it will be 

 brought into such close contact as to be air-tight. 

 In the middle is an opening A, which communicates 

 by a tube AB with a hollow cylinder or barrel, in 

 which a solid piston P is moved. The piston-rod C 

 moves in an air-tight collar D, and at the bottom of 

 the cylinder a valve V is placed, opening freely 

 outward, but immediately closed by any pressure 

 from without. There is thus a free communication 

 between the receiver R, the tube AB, and the ex- 

 hausting barrel BV. This communication extends 

 in the same manner to a second similar barrel XV. 

 When the piston CP is pressed down, and has passed 

 the opening at B, the air in the barrel BV will be 

 enclosed, and will be compressed by the piston. As 

 it will thus be made to occupy a smaller space than 

 before, its density, and consequently its elasticity, 

 will be increased. It will therefore press downwards 

 upon the valve V with a greater force than that by 

 which the valve is pressed upwards by the external 

 air. This superior elastic force will open the valve, 

 through which, as the piston descends, the air in the 



haml will be driven into the atmosphere. If the 

 piston be pushed quite to the bottom, the whole 

 air in the barrel wil^, be thus expelled. The mo- 

 ment the piston begins to ascend, the pressure of the 



air from without closes the valve completely. None 

 of the external air can enter; and, as the piston as- 

 cends, a vacuum is left beneath it ; but, when it rises 

 beyond the opening B, the air in the receiver R and 

 the tube AB expands, by its elasticity, so as to fill 

 the barrel BV. A second depression of the piston 

 will expel the air contained in the barrel, and the 

 process may be continued at pleasure. The com- 

 munication between the barrels and the receiver may 

 be closed by a stop-cock at G. It is evidently only 

 in consequence of the elasticity of the air that it ex 

 pands and fills the barrel, diffusing itself equally 

 throughout the cavity in which it is contained. The 

 operation of the machine depends, therefore, on the 

 elasticity of the air, and it is obvious that a perfect 

 vacuum cannot be formed by it in the receiver, as 

 only a part of the air is each time expelled, and a por- 

 tion must always remain after each depression of the 

 piston. The degree of rarefaction produced by the 

 machine may, however, be easily calculated. Sup- 

 pose that the barrel contains one third as much as 

 the receiver and tube together, and, therefore, that 

 it contains one-fourth of the whole air within the valve 

 V. Upon one depression of the piston, this fourth 

 part will be expelled, and three-fourths of the origi- 

 nal quantity will remain. One-fourth of this remain- 

 ing quantity will in like manner be expelled by the 

 second depression of the piston, which Ls equal to 

 three-sixteenths of the original quantity. By calcu- 

 lating in this way, it will be found that after thirty 

 depressions of the piston, only one 3096th part of the 

 original quantity will be left in the receiver. The 

 rarefaction may thus be carried so far that the elasti- 

 city of the air pressed down by the piston shall not 

 be sufficient to force open the valve. To show how 

 far the exhaustion has been carried at any particular 

 point of the process, a barometer-gauge is connected 

 with the machine. This is a glass tube, opening at 

 E into the receiver, and at F immersed in a cistern 

 of mercury. As the rarefaction proceeds, the mer- 

 cury rises from the pressure of the external air, and 

 indicates how far this pressure exceeds that from 

 within the receiver, that is, the degree of exhaustion. 

 Both pistons are worked by the wneel H and winch 

 Y, by means of the rack or tooth-work on the piston- 

 rods. When one piston is raised, the other is de- 

 pressed. The which is then turned in the opposite 

 direction, and the piston which had been raised is 

 depressed, and the other raised. When the rarefac- 

 tion of the air within the barrels is considerable, the 

 pressure of the atmosphere upon each piston is not 

 resisted from within, and therefore opposes its ascent. 

 But this pressure is not felt by the operator, as the 

 pressure upon one piston counterbalances that upon 

 the other. The elasticity of the air is proved by the 

 action of the machine. Its pressure is proved by the 

 great firmness with which the receiver is pressed upon 

 the plate SS during the rarefaction of the air within. 

 If any animal is placed beneath the receiver, and 

 the air exhausted, he dies almost immediately ; a 

 lighted candle under the exhausted receiver imme- 

 diately goes out. Air is thus shown to be necessary 

 to animal life and to combustion. A bell, suspended 

 from a silken thread beneath the exhausted receiver, 

 on being struck, cannot be heard. If the bell be in 

 one receiver, from which the air is not exhausted, 

 but which is within an exhausted receiver, it still 

 cannot be heard. Air is therefore necessary to the 

 production and to the propagation of sound. A shri- 

 velled apple or cranberry, placed beneath an ex- 

 hausted receiver, becomes as plump as if quite fresh. 

 They are thus shown to be full of elastic air. A great 

 variety of experiments may be made, which are very 

 interesting, but too numerous to be described. The 

 air-pump was invented by Otto de Guericke, burgo- 



