AIR AND VENTILATION 



91 



the windows, doors, ventilators and other apertures be thrown 

 open, the foul air will soon be removed and be replaced by fresh air. 

 On the return of the animals in the evening if the windows and 

 other ventilators are again closed the air will once more become foul 

 with excretory products. 



The temporary purification of the building that was effected 

 during the day is not an efficient substitute for true ventilation, 

 but it is a method that is frequently adopted. Ventilation in the 

 correct sense is a continuous, not an intermittent process. 



Either natural or mechanical means may be employed to 

 ventilate buildings. With the former method advantage is 

 taken of the natural gas laws that govern the mixing and move- 

 ment of the air, and though various contrivances may be used to 

 facilitate the air exchange, the adoption of such devices does not con- 

 stitute a mechanical or artificial system. A mechanical method of 

 ventilation is one in which mechanical power is used to force air 

 into or to draw air out of a building. In many buildings mechanical 

 ventilation is introduced to aid a natural system. 



NATURAL VENTILATION. The two chief agents that keep the air 

 pure and healthy are the movements of volumes of air of unequal 

 temperature and the diffusion of gases of different densities. In a 

 building the carbon dioxide and methane excreted by animals 

 become diffused among the gases normally present ; they do not con- 

 centrate around their source of production, but, by diffusion, are 

 constantly diluted. The diffusion of gases is, therefore, an import- 

 ant aid toward ventilation by ensuring a certain uniformity in the 

 composition of the air in a building. Chemical examinations of 

 the air in stables and byres have shown that the percentage of CO 2 

 differs very little in the various sections of these buildings. 



As ordinary building bricks are porous they permit of diffusion 

 taking place through them, but such action is slow and ceases 

 altogether if the brick walls are faced with cement or satu- 

 rated with water. For practical purposes the actual passage of air 

 into and from a building may be considered to depend upon the 

 movements of volumes of air of different temperatures. 



It has been stated in the section dealing with meteorology that a 

 wind is the result of the upward passage into a higher altitude 

 of heated and expanded air, the wind being a body of cooler 

 air flowing in to fill the space thus created. Exactly the same 

 process takes place in a building where there are animals whose 

 body temperatures are greater than that of the air surrounding 

 them. As the air becomes warmed it expands and rises to a higher 

 level. If, therefore, provision is made for the escape of this heated 



