RESPIRATION AND EXCRETION 



387 



index by which we may prevent poisoning. Air containing from 

 6 to 8 parts of carbon dioxide to 10,000 parts of air is bad ; while 

 from 12 to 14 parts in 10,000 makes a very dangerous amount. 

 Among the factors which take oxygen from the air in a closed 

 room and produce carbon dioxide are burning gas or oil lamps, 

 stoves, the presence of a number of people, etc. 



Proper Ventilation. — Ventilation consists in the removal of 

 air that has been used, and the introduction of a fresh supply to 

 take its place. If we remember that warm air is lighter than cold 

 air, and carbon dioxide is heavier than air, we can see that ventila- 

 tion outlets should be on the level of the floor. The inlets should 

 be near the top of the room, especially in houses heated by any 

 method of direct radiation, such as steam or hot water. A good 

 method of ventilation for the home is to place a board two or three 

 inches high between the 

 lower sash and the frame 

 of a window. An open 

 fireplace in a room aids 

 in ventilation because of 

 the constant draft up 

 the flue. 



Sweeping and Dusting. 

 — It is very easy to dem- 

 onstrate the amount of 

 dust in the air by follow- 

 ing the course of a beam 

 of light in a darkened 

 room. We have already 

 proved that spores of 

 mold and yeast exist in 

 the air. That bacteria 

 are also present can be 

 proved by exposing a sterilized gelatin plate to the air in a 

 schoolroom for a few moments.^ 



Plate culture exposed for ti\-u ininutcM in a school 

 hall where pupils were passing to recitations. 

 Each spot is a colony of bacteria or mold. 



' Expose two sterilized dishes containing culture media ; one in u room being 

 swept with a damp broom, and the other in a room which is being swept in the usual 

 maimer. Note the formation of colonies of bacteria in each dish. In which dish 

 does the most growth take place? 



