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EXPERIMENTAL GENERAL SCIENCE 



school-rooms, therefore, should be at least 50 per cent. Some 

 interesting phases of humidity are encountered when dwellings 

 are heated by furnaces. When the air is taken in from out- 

 doors, as it commonly is, the heat of the fire causes it to greatly 

 increase in volume, but the moisture it contains is not increased 

 in amount and there is therefore much less per cubic foot than 

 in the outside air. To remedy this, moisture is usually added 

 to the air by evaporating water into it. Even when such 

 means are used the air in our dwellings is seldom moist enough. 



Not only do we feel warmer in moist 

 air above a certain temperature, but 

 the membranes of the throat and 

 other respiratory passages are kept 

 in better condition and thus colds 

 and sore throats are avoided. 



95. The Hygrometer. A device 

 for measuring the moisture in the air 

 is called a hygrometer. One of the 

 simplest, called the hair hygrometer, 

 is made of a single long hair from 

 which all oil has been removed by 

 soaking it in ether. When exposed 

 to moist air this absorbs moisture and 

 contracts, and when it dries, it length- 

 ens again. If passed over a tiny 

 FIG. 35. Wet- and dry-bulb pulley and kept taut by a small 



weight, its shortening and lengthen- 

 ing may be made to move a hand on the pulley and so indicate 

 the relative amount of moisture in the air. The instrument 

 most frequently used is called a psychrometer. It consists of 

 two thermometers that read exactly alike mounted together. 

 One thermometer is so arranged that its bulb may be kept 

 wet. As the moisture evaporates, the temperature of this 

 thermometer is lowered, and if the air is dry, the evaporation 



