108 THE ATMOSPHERE AND ITS SERVICE TO MAN 



In the interest, therefore, of the conservation of health, 

 as well as of fuel and of furniture, open vessels of water 

 (humidifiers) should be kept on stoves, radiators, or regis- 

 ters, in order to keep the air of living rooms moist. Hang- 

 ing up cloths, the ends of which are in pails of water, will 

 serve the purpose even better, because they increase the 

 surface from which evaporation takes place and thus furnish 

 more water to the air in less time. (Figure 53.) There are 

 many patented devices for humidifying, but the principle on 

 which all of them are constructed is the same as that of the 

 homemade humidifier. A temperature of between 65 F. 

 and 70 F. will make a room comfortable if there is sufficient 

 moisture in the air. 



Weight of Air. Experiment 41. Into a five-pint bottle insert 



a tightly fitting rubber stopper through which a glass tube extends. 



To the outer end of the glass tube tightly fit a thick- 



r flf walled rubber tube of sufficient length for the attach- 

 ment of an air pump. Put a Hoffman's screw upon the 

 rubber tube. (Figure 54.) See that all connections are 

 air-tight. Weigh carefully the apparatus as thus 

 arranged. Now attach the rubber tube to an air pump 

 and extract the air from the bottle. When all the air 



FIGURE 54 ^at can be exhausted has been removed, close the 

 rubber tube tightly with the Hoffman's screw and weigh 

 again. Unclamp the Hoffman's screw and allow the air to enter 

 the bottle. The weight should be now the same as at first. Or, 

 instead of weighing a bottle of air, weigh an incandescent light bulb. 

 Make a hole in it with a blowpipe and weigh again. Is the weight 

 now the same as before ? 



We have found by the previous experiment that air has 

 weight. With the apparatus used it was impossible to 

 tell exactly the weight of the air extracted or to determine 

 the weight of a definite volume of the air. If we had been 



