142 RESPIRATION. 



affords us, on a small scale, an illustration of the mutual 

 dependence of the two great kingdoms of nature; as well 

 as of those compensating changes which are taking place 

 on such a grand scale in the world about us. 



42. Ventilation. Since the external atmosphere, as 

 provided by nature, is always pure, and since the air in our 

 dwellings and other buildings is almost always impure, it 

 becomes imperative that there should be a free communica- 

 tion from the one to the other. This we aim to accomplish 

 by ventilation. As our houses are ordinarily constructed, 

 the theory of ventilation, "to make the internal as pure as 

 the external air," is seldom carried out. Doors, windows, 

 and flues, the natural means of replenishing the air, are 

 too often closed, almost hermetically, against the precious 

 element. Special means, or special attention, must there- 

 fore be used to secure even a fair supply of fresh air. This 

 is still more true of those places of public resort, where 

 many persons are crowded together. 



43. If there are two openings in a room, one as a vent 

 for foul air, and the other an inlet for atmospheric air, 

 and if the openings be large, in proportion to the number 

 of air consumers, the principal object will be attained. 

 Thus, a door and window, each opening into the outer air, 

 will ordinarily ventilate a small apartment; or a window 

 alone will answer, if it be open both above and below, and 

 the open space at each end be not less than one inch for 

 each occupant of the room, when the window is about a 

 yard wide. The direction of the current is generally from 

 below upward, since the foul, heated air tends to rise; 

 but this is not essential. Its rate need not be rapid ; a 

 " draught," or perceptible current, is never necessary to 

 good ventilation. The temperature of the air admitted 

 may be warm or cold. It is thought by many that if the 



42. Character of the external air? Of the air in our dwellings ? What becomes 

 imperative ? Imperfect ventilation of our dwellings ? 



43. What hints are given for the ventilation of our dwellings? 



