120 THE ATMOSPHERE AND ITS SERVICE TO MAN 



flow again. If the water, once started, is allowed to flow, it will 

 empty the bottle to the end of the glass tube. Any bent tube 

 arranged in this way, with one arm longer than the other, is called 

 a siphon. 



In Experiment 50, the water in the siphon was pressed 

 outward from the bottle by atmospheric pressure minus the 

 weight of the column of water in the short arm. It was 

 pressed toward the inside of the bottle by atmospheric pres- 

 sure minus the weight of the column of water in the long 

 arm. The atmospheric pressure was practically the same in 

 both cases, but the weight of the, water column in the short 

 arm was less than that of the water column in the long 

 arm. The pressure acting outward was therefore greater 

 than that acting inward and the water flowed out of the 

 bottle. The siphon continued to flow as long as the in- 

 equality of pressure was maintained. When the atmospheric 

 pressure was shut off by the insertion of the sealed glass 

 tube, the water of course stopped flowing. 



Vacuum Cleaners. Experiment 61. Allow a beam of light 

 to enter a darkened room through a small hole in a curtain. Note 

 as carefully as you can the different things in the air that the 

 beam of light reveals. 



In the preceding experiment we observed that the air 

 contained something more than the gases and moisture 

 which we have learned are in it. There are many solid 

 particles floating in the air. There were little shreds of 

 cloth and paper, pieces of dust and soot, and many other 

 things. The beam of light, however, did not reveal every- 

 thing that was floating in the air. There were many living 

 organisms, tiny plants (bacteria) , too small to be seen except 

 by the aid of a high-power microscope. 



These minute living things are scattered all through the 



