RESPIRATION 



199 



and liberate oxygen. Taking advantage of this fact, we may 

 so arrange aquaria with fishes and sea-plants, in their proper 

 combinations, that each may supply the needs of the other, 

 and the water may seldom require to be renewed. This affords 

 us, on a small scale, an illustration of the grand circle of 

 changes taking place in the air about us, and also of the har- 

 monious dependence of the two great kingdoms of nature. 



42. Ventilation. Since the external atmosphere, as pro- 

 vided 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 communication 

 from the one to the other. This we aim to accomplish by venti- 

 lation. 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 spe- 

 cial attention, must 

 therefore be used to se- 

 cure even a fair supply 

 of fresh air. This is 

 still more true of those 

 places of public resort, 

 where large numbers 

 of persons are crowded 

 together. (See House- 

 Drainage, Appendix.) 



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 



FIG. 53. Showing manner of ventilating by inserting 

 strip of wood beneath lower sash of window 



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

 tive ? Imperfect ventilation of our dwellings ? 



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



