Mingling of Molecules 



281 



FIG. 152. Filling the barometer tube with mercury. 



the tube upright with your hand or by clamping it to a 

 ring stand. Hold a yardstick or meter stick beside the tube, 

 remembering to keep the tube straight up and down. Meas- 

 ure accurately the height of the mercury column from the 

 surface of the mercury in the cup. Then go to the regular 

 barometer hanging on the wall, and read it. 



The reason your barometer may not read exactly the same 

 as the expensive laboratory instrument is that a little air 

 and water vapor stick to the inside of the tube and rise into 

 the " vacuum " above the mercury ; also, the tube may not 

 be quite straight up and down. Otherwise the readings 

 would be the same. 



Of course you understand what holds the mercury 

 up in the tube. If you could put the cup of mercury 



