PROPERTIES OF GASES 



103 



liquid. The end of the long arm must be lower than the 

 surface of the liquid that is to be moved. When the ends 

 of the siphon are uncovered, liquid falls from the long arm. 

 This would tend to cause a vacuum above, but the pressure 

 of air upon the surface of the liquid causes it to rise in the 

 short arm, as soon as the pressure is lessened at the other 

 end. The flow continues until the liquid reaches the same 

 level on both sides, or until the surface of the liquid is as 

 low as the end of the short arm of the siphon. 



112. Pumps. We are all familiar with the operation of 

 taking lemonade through a straw. We say we "suck it up." 

 What we do is to draw the air from the tube, creating there 

 a partial vacuum. (An old saying is that "Nature abhors 

 a vacuum." This means that in natural conditions a vacuum 

 does not exist.) Just as soon as 

 the pressure is diminished in the 

 tube, the pressure of air upon 

 the liquid pushes it up through 

 the tube to the mouth. If air 

 is allowed to come into the top 

 of the tube for an instant, the 

 liquid falls back into the glass. 



This method of drawing 

 lemonade from a glass is similar 

 to the method used in drawing 

 water from a well by a pump 

 which is sometimes called the 

 suction pump. This consists of 

 a pipe of metal or wood placed 

 upright, with the lower end near 



the bottom of the well. A piston works up and down in the 

 upper and larger part of the pipe (called the cylinder) , fitting 

 it water-tight. An opening through the piston is closed by 

 a hinged cover, (called a valve) which opens upward. There 

 is also a valve opening upward in the pipe below the piston. 



FIG. 50. A PEN FILLER 



1. Compare the situation in 

 A with the first step in the 

 process of sucking liquid through 

 a straw. 2. Compare B with 

 the next step. 3. Under what 

 conditions would ink rise to the 

 top of the tube? 



