20 GENERAL SCIENCE 



extreme cold or dampness is excuse for sleeping temporarily 

 shut away from an abundance of outside air. An open but 

 well-screened sleeping porch, with a warm room to dress in 

 when the weather is cold, makes quite an ideal condition. 



During the day the ordinary activities of life render un- 

 necessary any special attention to ventilation save where large 

 numbers of people are together as in public halls, schools, 

 and factories. In these places there is likelihood of oxygen 

 starvation. The air, too, soon becomes laden with moisture 

 from perspiration, and from water in the respired air. Dis- 

 ease germs may be given off into the air from any infected 

 person present. All this makes necessary the continuous 

 renewal of the indoors air. In cold weather air' from out of 

 doors should be warmed before it is allowed to spread through 

 the rooms. There should never be appreciable currents 

 (drafts) in any part of a room. The air should always be 

 free from dust, too, and should contain considerable mois- 

 ture, especially if it is furnace heated. A humidity of from 

 40 per cent to 50 per cent does not dry the mucous mem- 

 branes of the air passages, nor does it interfere with evapora- 

 tion of the perspiration. 



To maintain the circulation necessary to bring a continu- 

 ous supply of air into rooms, advantage is usually taken of 

 the fact that air when heated becomes less dense (lighter). 

 The air of a building may be kept in circulation by having 

 one portion warmer than other parts of it. The colder and 

 more dense air presses the warmer air onward and outward. 

 The chapters on heat and pneumatics (nu-mat'-iks) in 

 Physics usually include a study of air currents, and of sys- 

 tems of heating and of ventilating. The satisfactory ven- 

 tilation of the many rooms in large buildings used for offices, 

 stores, factories, or for schoolroom purposes, requires that 

 air be forced into them or withdrawn from them by some me- 

 chanical means, such as fans driven by electricity. This 



