DUST AND BACTERIA. 119 



the door very tight. Without any special provision for the 

 renewal of the air these cracks are the means of safety. 



In houses heated by furnaces, steam, or hot water, the 

 floor is likely to. be warmer from the escape of heat from 

 the heater itself, and from pipes or air ducts under the 

 floor. 



Double Windows. There is a very common misunder- 

 standing as to the cold felt near a window in cold weather. 

 It seems that air is entering ; but a little reflection will 

 show that even if the window were air-tight this effect 

 would be produced, for the air near the window is cooled 

 by losing heat to the outer air. The air next to the win- 

 dow, thus cooled, is heavier, and falls to the floor ; and if 

 there is any source of heat in the room, this cold air will 

 pass along the floor to that source of heat, up from the 

 heating body to the ceiling, and across the ceiling, and so 

 on around again. There may thus be currents without 

 any appreciable change in the quality of the air. It is 

 economy to use double windows and prevent the loss of 

 heat through the glass. So both economy -and comfort 

 suggest to us that we reduce as much as possible cracks 

 around doors and windows, use double windows, make ves- 

 tibules at entrances, and build special ducts by which fresh 

 air may enter, and heat it properly on its way in. 



DEAD DUST. 



The Air is washed by Rain or Snow. Every one 

 will recall how delightfully refreshing the air is after a rain 

 or a snowstorm. This is not due merely to the fact that 

 the air is cool. It is clean because it has been washed. 

 The rain and snow absorb a considerable amount of the 

 various impure gases that are in the air. But raindrops 



