PASSAGE OF OXYGEN THROUGH THE BODY ill 



way as the passage of the oxygen. Its ability to dissolve 

 in liquids and to enter into chemical combination varies as 

 the carbon dioxide pressure! This in turn varies with the 

 amount of the carbon dioxide, which is greatest at the cells 

 (where it is formed), less in the blood, and still less in the 

 lungs. Because of these differences, the blood is able to 

 take it up at the cells and release it at the lungs (Fig. 55). 

 Properties of Carbon Dioxide. Carbon dioxide is a color- 

 less gas with little or no odor. It is classed as a heavy 

 gas, being about one third heavier than air 2 (Fig. 56). 

 It does not support combustion, but on the contrary is 

 used to some extent to extinguish fires. 

 It is formed by the oxidation of carbon 

 in the body, and by the combustion of 

 carbon outside of the body. It is also 

 formed by the decay of animal and veg- 

 etable matter. From these sources it 



FIG. 56. Soap 



is continually finding its way into the bubble floating in a 

 atmosphere. Although not a poisonous vessel of carbon diox- 

 gas, carbon dioxide may, if it surround ide > illustrating the 



, , i i i , , i i r difference in weight 



the body, shut out the supply of oxygen , 



J rr j jo between air and car- 



and cause death. 3 



Carbon 

 dioxide 



between air and car- 

 bon dioxide gas. 



1 See footnote on oxygen pressure, page 109. 



2 The impression prevails to some extent that carbon dioxide, on account of its 

 weight, settles out of the atmosphere, collecting in old wells and at the floor in 

 crowded rooms. Any such settling of the carbon dioxide is prevented by the rapid 

 motion of its molecules. This motion not only prevents a separation of carbon 

 dioxide and air after they are mixed, but causes them to mix rapidly when they are 

 separated, if they still have surface contact. The carbon dioxide found in old wells 

 is formed there by decaying vegetable or animal matter. In rooms it is no more 

 abundant at the floor than in other parts. 



3 On account of the formation of carbon dioxide in places containing decaying 

 material, the descent into an old well or other opening into the earth is often a 

 hazardous undertaking. Before making such a descent the air should always be 

 tested by lowering a lighted lantern or candle. Artificial respiration is the only 

 means of restoring one who has been overcome by this gas (page 97). 



