480 WATER SUPPLY AND SEWAGE DISPOSAL 



Exercise 95. Removing the Air from Water 



Place a tumbler or flask filled with water under the receiver of the 

 air pump, first noting the temperature of the water. Begin pumping 

 the air from the receiver. While taking the first few strokes watch 

 carefully to see if any bubbles are rising through the water. Pump 

 out most of the air, watching constantly for bubbles of rising air. 



EXPLANATION. We all know that sugar or salt may be 

 dissolved in water; in much the same way air readily dissolves 

 in water. Since the atmosphere is always resting upon the 

 surface of water standing in an open vessel, it is always pos- 

 sible for air to be dissolved in the water. But is there a limit 

 to the amount of air which will be thus dissolved by a given 

 quantity of water? We know from experience that there is a 

 limit to the amount of sugar or salt which will dissolve in a 

 given quantity of water. After the water is " saturated" 

 with the salt or sugar, adding more of the solid simply means 

 that it will settle to the bottom of the liquid and remain there 

 undissolved. In the same way, a vessel of water standing open 

 to the air is soon "saturated" with dissolved air; it contains 

 all the air it is possible for it to contain under the given condi- 

 tions. From Ex. 95 what do you conclude is the effect of re- 

 ducing the pressure of the air upon the surface of the water? 



HENRY'S LAW. The amount of gas dissolved in water is 

 directly proportional to the pressure, that is t doubling the pres- 

 sure doubles the amount of gas which dissolves in a given quantity 

 of water. 



One cu. ft., or 1728 cu. in., of water at 0C. and at the pres- 

 sure of 15 Ib. per sq. in., that which the atmosphere exerts 

 at the sea level, dissolves about 45 cu. in. of air. By Henry's 

 Law we see that doubling this pressure would cause about 

 90 cu. in. of air to be dissolved in each cubic foot of water, or 

 reducing the pressure to 5 Ib. to the sq. in. reduces the amount 

 of dissolved air to 15 cu. in. per cu. ft. 



The need of pumping more air into the tank occasionally 

 to prevent it from becoming " water-logged" is now' evident. 

 The water in the well or cistern is under 1 atmosphere of 



