THE ATMOSPHERE. 



continued to any required extent, a literal and absolute vacuum 

 can never be produced, because some quantity of air, however 

 small, must always remain in the vessel E. After every stroke 

 of the piston, nine-tenths of the air which is in the vessel 

 before the stroke remains in it. Now it is evident, that if wo 

 successively subtract one-tenth of any quantity, we must always 

 have some remainder, however long the process be continued : 

 and the same will be true, whatever proportion be thus continually 

 subtracted. 



27. Nevertheless, although an absolute vacuum cannot be 

 obtained by such means, we can continue the process until the 

 rarefaction shall be carried to any required extent. 



In practice, the stop-cocks D and c are replaced by valves. A 

 valve is placed at D which, opening outwards, is forced open by 

 the elasticity of the air compressed under the piston when 

 depressed, but is kept closed by the external pressure of the 

 atmosphere when the piston is raised. The valve at c opens 

 upwards, and is opened by the elasticity of the air in E when the 

 piston is raised, and kept closed by the elasticity of the com- 

 pressed air in the cylinder when the piston is depressed. Instead 

 of placing a tube and valve at B, it is usual to make the valve in 

 the piston itself, opening upwards; but the action is still the 

 same. An exhausting syringe, therefore, may be shortly described 

 to consist of a cylinder with two valves, one in the bottom, 

 opening upwards, and one in the piston, also opening upwards. 

 When the piston is drawn upwards, the valve in the bottom of 

 the cylinder is opened by the pressure of the air under it, and the 

 air passes through it. When the piston is driven downwards, the 

 valve in the piston is opened by the elasticity of the air com- 

 pressed under it, which rushes through it. 



28. The air-pump is an apparatus consisting usually of two 

 exhausting syringes, B B', fig. 6, mounted so as to be worked by 

 a single winch and handle, as represented at D, and communi- 

 cating by a common pipe T with a glass vessel E, in which may bo 

 placed the objects of experiment. The vessel E, called a receiver , 

 has an edge s, ground smooth, resting upon a plate, also ground 

 smooth, and kept in air-tight connection with it by being smeared, 

 with hog's lard. A stop-cock c is provided in the pipe T, by which 

 the communications between the receiver E and the syringes can 

 be made and broken at pleasure. Another stop -cock is provided 

 elsewhere, by which a communication can be made at pleasure 

 between the interior of the receiver E and the external air. To 

 indicate the extent to which the rarefaction is carried from time 

 to time by the operation of the syringes, a mercurial gauge 

 ir Q M is provided, constructed in all respects similar to a 

 110 



