ELASTICITY OF AIR. 47 



removed. If we suppose the magnitude of this hole, together with whatever 

 space may remain unfilled between the lower surface of the piston and the 

 bottom of the barrel, to be the ten thousandth part of a solid inch, then the 

 valve V 7 will cease to act when the air which fills the barrel, the piston being 

 at the top, is such that if condensed into the ten thousandth part of an inch, its 

 elastic force will exceed the atmospheric pressure by a quantity less than the 

 force required to open the valve V. This source of imperfection will evidently 

 be diminished by diminishing the depth of the aperture below the valve V 7 , and 

 by increasing the size of the cylinder ; for if the air in the barrel be as many 

 times rarer than the external atmosphere, as the magnitude of the barrel is 

 greater than the magnitude of the space below the valve V 7 , then this air, 

 when condensed into that space, will exert a pressure equal to that of the at- 

 mosphere. Suppose the barrel contains ten cubic inches of air, and that the 

 magnitude of the hole is the hundredth part of a cubic inch, then the magnitude 

 of the cylinder will be one thousand times the magnitude of the space which 

 remains between the valve V' and the bottom of the barrel, when the piston is 

 pressed to the bottom. Consequently the process of rarefaction would be de- 

 duced, until the air in the receiver would be rendered one thousand times rarer 

 than the external atmosphere. 



The vessel R, being connected with a tube furnished with a stop-cock C, 

 may be detached from the syringe, together with the stop-cock, by unscrewing 

 the tube C ; and if the stop-cock be previously closed, the interior of the vessel 

 will continue to contain the rarefied air. 



In various branches of physical science, inquiries continually arise, re- 

 specting qualities and effects of material substances, which are subject to con- 

 siderable modification by the pressure or other qualities of the air which sur- 

 rounds them ; and it is often necessary in such investigations to discover what 

 these qualities and effects may be, if the substances were not exposed to the 

 mechanical pressure or other effects consequent upon the presence of the at- 

 mosphere. Although we do not possess any means of removing altogether the 

 presence of this fluid, yet, from what has been already stated, it is plain that 

 it may be so attenuated in an enclosed chamber, such as the vessel R, that 

 these effects may be diminished in intensity to any degree which experimental 

 inquiry may demand. 



With these views it is necessary, however, not only to be able to introduce the 

 substances which are submitted to experimental investigation, into the chamber 

 in which the rarefaction has been accomplished, but also to be able to observe 

 them when so situated. The latter purpose could be accomplished by construct- 

 ing the receptacle R, of glass ; but still it would be necessary to have access to | 

 the interior, and to construct it of a convenient form to receive the subjects of ex- i 

 periment, and even in many cases to be able to manipulate or produce changes | 

 of position on the object thus enclosed. 



For these purposes, the form of the vessel R, and the mode of connecting it | 

 with the syringe, must be somewhat changed, and the arrangement which is ' 

 given, in order to adapt them thus to all the exigencies of experimental inves- 

 tigation, is called the air-pump, an instrument which we will now proceed to 

 explain. 



THE AIR-PUMP. 



The vessel in which the rarefaction is produced by an air-pump, is called a 

 receiver, and is usually constructed of glass in a cylindrical form, with an 

 arched or round top, furnished with a ball as a convenient handle. A section, 

 R, of this is represented in fig. 4. The mouth, or lower part, is open, and it 



