THE MECHANICS OF VENTILATION. 475 



;and uoontimo. These results are immediate depression of toinperature, iii 

 some cases sufficient to fall below the dew point, hence moisture is actually 

 deposited as dew about the room, and hence colds and sickness of teachers 

 and pupils generally. The same thing will occur not only in school rooms, 

 but in any illy-ventilated apartment where a large number of people being 

 congregated, windows are suddenly opened for the admission of fresh air. 



The question of humidities is somewhat complicated. It will be emi- 

 nently proper, therefore, to consider briefly the principles involved; and 

 in making such consideration Deschanel's Physics will be drawn upon. 

 With this general acknowledgement such use is made of the excellent work 

 in question as may be necessary to the proper elucidation of the subject in. 

 hand. 



The condition of the air as regards moisture involves two distinct ele- 

 ments : (1) the amount ot vapor present in the air, and (2) the ratio of this 

 to the amount which would saturate the air at the actual temperature. There 

 are two important laws bearing upon the subject: 



1. The weight of vapor which Avill enter a given space is the same 

 whether this space is empty or filled with a gas. 



2. When a gas is saturated with a vapor the actual tension of the mix- 

 ture is the sum of the tensions due to the gas and the vapor separately ; that 

 is to say, it is equal to the tension which the gas would exert if it alone 

 occupied the whole space plus the maximum tension of vapor for the tem- 

 perature of the mixture. 



The word tension means in this connection the force acting to produce 

 ■expansion, as opposed to pressure, or the force tending to produce compres- 

 sion, or we may say the tension decreases the density of a gas while pressure 

 increases that element. 



Eelative humidity is defined as the weight of aqueous vapor in a given 

 volume of air, expressed as a percentage of the weight of vapor at saturation 

 which would occupy the same volume at the actual temperature. 



When air containing water in form of vapor is gradually cooled at con- 

 stant pressure, its capacity for vapor gradually decreases until the point of 

 saturation is reached. Any farther reduction of temperature is accompanied 

 by a deposit of moisture. This point of deposit is called the dew point, the 

 relation of which to the actual temperature is shown by the humidity tables 

 below. 



There is a popular idea that air loses its moisture by heating. In fact, 

 since moisture exists in the air, so far as known in the form ot vapor and 

 not mechanically suspended, the only way the air can lose moisture is by 

 reduction of temperature to the dew point, and by consequent deposition in 

 the form of dew. The effect of heating air is to increase the capacity for 

 moisture, accompanied by decrease of relative humidity, an apparent dryness 

 may be thus attained, although the absolute humidity remains the same as 

 previous to the heating. * * * ''^ * * 



According to General Morin, the amount of air to be changed every hour, 

 in order to keep within the limits of good ventilation, is as follows : 



