80 NON-METALS AND THEIR COMBINATIONS. 



Cold water absorbs at the ordinary pressure about its own 

 volume of carbon dioxide, arid much larger quantities under an 

 increased pressure (soda water). 



Carbon dioxide is not combustible, and not a supporter of 

 combustion; on the contrary, it has a decided tendency to ex- 

 tinguish flames, air containing one-fourth of its volume of carbon 

 dioxide being unable to support the combustion of a candle. 

 Whilst not poisonous when taken into the stomach, carbon di- 

 oxide acts as a direct poison when inhaled. 



Common atmospheric air contains about 4 volumes of carbon 

 dioxide in 10,000 of air, or 0.04 per cent. In the process of 

 respiration this air is inhaled, and a portion of the oxygen is 

 absorbed in the lungs by the blood, which conveys it to the 

 different portions of the animal body, and receives in exchange 

 for the oxygen a quantity of carbon dioxide, produced by the 

 union of a former supply of oxygen with the carbon of the 

 different organs to which the blood is supplied. 



The air issuing from the lungs contains this carbon dioxide, 

 which amounts to about 4 volumes in 100 of exhaled air, which 

 is 100 times more than contained in fresh air. It is not advis- 

 able to breathe for any length of time air containing more than 

 0.1 per cent, of carbon dioxide; in air containing 0.5 per cent, 

 most persons are attacked by headache, still larger quantities 

 produce insensibility, and air containing 8 per cent, of carbon 

 dioxide causes death in a few minutes. 



As exhaled air contains from 3.5 to 4 per cent, of carbon di- 

 oxide, it is entirely unfit to be breathed again. The total 

 amount of carbon dioxide evolved by the lungs and skin of a 

 grown person amounts to about 0.7 cubic foot per hour. Hence 

 the necessity for a constant supply of fresh air by ventilation. 

 This becomes the more necessary where an additional quantity 

 of carbon dioxide is supplied by illuminating flames. A burn- 

 ing candle, for instance, produces nearly as much carbon dioxide 

 as an adult man, and an ordinary gas flame as much as five 

 persons. 



Many processes by which carbon dioxide is constantly pro- 

 duced in nature have been mentioned above, and we might 

 assume that the amount of 0.04 per cent, of carbon dioxide con- 

 tained in the atmospheric air would gradually increase. This, 

 however, is not the case, because the plants, and more especially 



