56 



It was formerly thought that foul air, that is air that has been 

 breathed, was more dense than fresh air, because part of the oxygen of 

 the latter is replaced by carbon dioxide in the lungs, and carbon dioxido 

 is denser than oxygen, and consequently tliat expired air tended to fall 

 and foul air to accumulate at the floor of a room, so that for the best 

 results the removal of air should be from near the floor. This reasoning 

 overlooked the fact that oxygen is also replaced by water vapor in tlb' 

 lungs, and water vaiwr is lighter than oxygen ; also that the expired air 

 is at a higher temperature than the air of the room and <in this account 

 less dense. This error is no longer generally made in the discussion of 

 the principles although often in practice. As will l>e shown, expired air 

 is actually lighter than fresh air under ordinary ventilation conditions, 

 and therefore tends to rise and accumulate near the ceiling. This is as- 

 sisted by the natural upward movement of air in a building warmer than 

 its surroundings, as in a flue, and further by upward currents in the neigh- 

 borhood of any body warmer than its immediate surroundings, such as a 

 stove, a burning lamp or gas jet or electric light, or even the body of a 

 person. That foul air tends to accumulate near the ceiling is very evi- 

 dent to those occupying the gallery of a crowded auditorium. 



An exiieriment to test this upward movement of respiretl air was 

 made by the writer in a class room about 27 by 30 feet and Ui feet high. 

 The room temperature was 24° C. (75° V.). and the ontdtx»r temperature 

 10° C. (HO" F. ) ; the moisture in the air of the room as shown by a Mltt- 

 hof hygrometer was between HO and (>0 per cent, of saturation. The win- 

 dows and door and a ventilator were closed during the ixn'iod of exi)eri- 

 ment and the only source of artificial heat in the room was a vertical 

 steam pipe, the radiator being shut off by the automatic thermostat. 



The room was oeeupied by 2(i adults for ".() minutes and was then 

 unoccupied for 10 minutes immediately before the jieriod of expcrinicnt. 

 which also lasted HO minutes, .">(> adults being present, seated. 



('arl)()n dioxide was (Iclerniined in llie air with a Tjinge air tester, 

 samples being taken alternately from within (t inches of the ceiling and 

 tlie floor, tlirongh tultes, and analyzed on a table, near the center of the 

 room, 'i'lie annl.\li<ai nirthod consisted in f<)ri-ing the air through a stand- 

 ard solution of sodium carbonate colored pink with pIienoli)]itliMhMn. by 

 squeezing a rubber bulb until the pink color disappeared, the number of 

 Sipieezes iieing coiuited. and ranging in tliis experiment from S (o r>, fresh 

 ontdooi- air i-cqniring is siiiU'fZfs witii tlie aiMiaratus used. 



