THE ATMOSPHERE. 





Birdcage fountains are constructed on the same principle. 



The peculiar gurgling noise produced in decanting wine arises 

 from the pressure of the atmosphere forcing air into the interior of 

 the bottle to replace the liquid which escapes. 



23. The common syringe by which air is withdrawn from or 

 condensed in any vessel derives its efficacy altogether from this 

 property of the elasticity of air. 



The instrument is called an exhausting or condensing syringe, 

 according as it is adapted to extract air from a vessel, or to force 

 air into it. 



24. To explain the principle of the exhausting syringe, let A B 

 (fig. 5) represent a cylinder having a solid piston r, moving- 

 air-tight in it. Let c be a tube proceeding 

 from its lower end, furnished with a stop-cock 

 c, and let B be another tube furnished with a, 

 stop-cock D. Let the tube c be screwed upon 

 any vessel such as n, from which it is desired 

 to extract the air. 



If the piston be now raised in the cylinder, 

 the cock D being closed and the cock c being- 

 open, the air in B, will necessarily expand, in 

 virtue of its elasticity, so as to fill the enlarged, 

 space provided by raising the piston. The 

 air which previously filled the vessel R and the 

 connecting tube will, in fact, now fill these, 

 and also the enlarged space in the cylinder. 

 When the piston is brought to the top of the 

 cylinder, let the cock c be closed and the cock 

 D be opened. Upon driving down the piston, 

 the air which fills the cylinder will be expelled 

 from the tube B through the open stop-cock D. 

 When the piston has reached the bottom of 

 the cylinder, let D be closed and c opened, 

 and let the same process be repeated ; the air filling the vessel K 

 will, as before, dilate itself, so as to fill such vessel and the 

 cylinder. The cock c being again closed, and D opened, and the 

 piston driven down, the air which fills the cylinder will be again 

 expelled. This process being continued, any desired quantity of 

 air can be taken out of the vessel n and expelled into the 

 atmosphere. 



It is evident that the escape of the air from B, into the cylinder 

 is effected in virtue of its elasticity ; while its escape from the 

 stop-cock D into the atmosphere is effected in virtue of its com- 

 pressibility. 



25. It is easy to explain the rate at which the air is drawn 

 108 



