MOISTURE IN THE AIR 



101 



water surface amounts to perhaps eight feet per year. This 

 means that the energy of the sun evaporates about five hundred 

 pounds of water from every square foot of the surface every 

 year. In the polar latitudes the amount 

 of evaporation is perhaps a tenth of that 

 in the tropics. 



From every water surface on the globe, 

 however, a large amount of water is 

 evaporated each year. 



Effect of Temperature on the Capacity 

 of the Air to Hold Moisture. Experi- 

 ment 37. Take a liter flask and put into it 

 just sufficient water to make a thin film on the 

 inside of the flask when shaken around. Now 

 warm the flask gently, never bringing its tem- 

 perature near to the boiling-point, until the 

 water disappears from the inside and the flask 

 appears to be perfectly dry. Having tightly 

 corked the flask, allow it to cool. The flask 

 appears dry when warm and on account of having been corked 

 tightly no moisture could have entered it. The air in the flask 

 was perfectly transparent both before and after heating. The film 

 of water around the inside of the flask was taken up by the air 

 when it was warmed but the moisture reappeared when the flask 

 was cooled. 



Experiment 38. Fill a bright tin dish or glass beaker with ice 

 water and after carefully wiping the outside allow it to stand for 

 some time in a warm room. Can water go through the sides of 

 the dish? Does the outside of the dish remain dry? If water 

 collects upon it, from where does the water come ? See if the same 

 results will follow if the water within the dish is as warm as or 

 warmer than the air in the room. 



Experiment 39. Partially fill a dish or beaker like that in the 

 previous experiment with water having a temperature a little 

 warmer than that of the room. Gradually add pieces of ice, con- 



BY RAPID EVAPO- 

 RATION 



