98 Domestic Science 



mouthed bottle, fitted with a tight-fitting rubber stopper, 

 bored with two holes. One of these holes admits a 

 glass tube T, similar to those used in the preceding 

 experiment, filled with, and inverted over, mercury in 

 the bottle, and the other a short length of glass tubing 

 D, bent at a right angle. The outer end of D is attached 

 to an air-pump or to a water-pump as commonly used 

 by chemists to hasten filtration. The level of the 

 mercury in the tube is noted and the pump is gently 

 worked. The height of the column of mercury in the 

 tube will be seen to diminish gradually as the air is 

 removed by the action of the pump from the vessel. 

 This diminution will continue till the limit of the 

 efficiency of the pump is attained, when the mercury 

 will remain at a more or less constant level. Pinch 

 the rubber connection to the pump tightly and dis- 

 connect the pump from the other end of the connection. 

 Open the rubber tube by releasing the pressure of the 

 fingers on it. 



Exercise for Student. 



Explain as clearly as you can why the mercury level in the tube 

 changes during this experiment. 



59. The original experiment with the tube of 

 mercury is due to Torricelli and the experiment is often 

 referred to as the " Torricellian experiment ", while the 

 apparatus is called the "Torricellian barometer" 

 (Greek, baros, weight, and metron, measure). A baro- 

 meter is then an instrument whereby the pressure of the 

 atmosphere is measured by balancing it against the 

 action of another force the force in this case being the 

 weight of the column of mercury. Other forms of this 

 instrument exist, of which the U~tube barometer, or 

 siphon barometer, is the most commonly occurring 



