124 FIRST YEAR SCIENCE 



the pump is not being worked any more swiftly than at 

 first. It is due to the greater compression of the air. As 

 this compression increases, the heating increases, the effect 

 of friction in a well-oiled pump being of small value. 



60. Effect of Temperature on the Capacity of the Air to hold 

 Moisture. Experiment 71. 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 appeared 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 re- 

 appeared when the flask was cooled. 



Experiment 72. 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 happen if the 

 water within the dish is as warm as or warmer than the air in the room. 



Experiment 73. 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, continually stirring 

 with a chemical thermometer. Note the temperature at which a mist 

 begins to appear upon the outside of the dish. When the mist has 

 appeared, add no more ice but stir until the mist begins to disappear. 

 Note this temperature. Take the average of these two temperatures. 

 This average is probably the temperature at which the mist really 

 began to form. This temperature is called the dew point. 



When we wish to dry clothes, we place them in a warm 

 room or in the sunshine. Soon we find that the water has 

 left the clothes. It must have gone into the air. It 

 would thus appear that when the temperature of th.e air is 

 raised, it has the capacity of taking up more moisture than 

 when it is cold. The previous experiment has shown this, 



