VENTILATING THE HOME 25 



be 100 per cent. If it holds one half as much moisture 

 as it could contain, the humidity is said to be 50 per cent. 

 Outdoors the humidity ranges from 65 to 75 per cent or 

 more. Indoors during the winter in the ordinary heated 

 room, it is found that the humidity is very low, ranging 

 from 20 to 30 per cent. 



During the winter the air taken into the heating system 

 from outdoors is very cold, and as it is heated, its power to 

 hold more moisture increases rapidly, doubling for about 

 each change of 20 degrees : thus air which at a temperature 

 of 30 degrees may be half saturated, at a temperature of 

 70 degrees will be only about one eighth saturated, and hence 

 be very dry and possess great absorbing power. This 

 humidity may be raised by introducing water into the hot- 

 air chamber in the cellar. Many of the water pans often 

 found in hot-air furnaces are worthless, because they do not 

 add enough moisture to affect the humidity appreciably. 



In order to furnish enough moisture for an ordinary house, 

 from one to two quarts of water should be evaporated hourly. 

 There are now being sold on the market instruments called 

 humidifiers, which can be used to supply the necessary water. 



DEMONSTRATION 8 



Purpose. To find the humidity of the room. 



Apparatus. Two thermometers, or a sling psychrometer. 



Directions, i. Fasten a piece of soft muslin cloth around the 

 bulb of one of the thermometers, and allow the other end of the 

 cloth to hang in a bottle filled with water. Hang another 

 thermometer beside this. Fan the bulbs vigorously for a short 

 time, then look at the reading of the wet bulb. Continue to 

 fan until the mercury in the wet bulb ceases to go any lower. 

 Then take the reading of both the thermometers. A slight 

 difference means a moist air. A large difference means a dry 

 air. The per cent of humidity may be found from the table in 

 the appendix, page 555. For example, if the difference between 

 the two thermometers is 16 degrees, and the reading of the dry 



