212 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



observations show an average not exceeding 0.0317 per cent, even at 

 night, and a general mean of about 0.0308, or 3.08 volumes in 10,000. 

 All the most recent works on hygiene, however generally accurate, 

 repeat this error. 



Considering the value of small quantities in these measurements, 

 especially where they affect human life, it is most desirable that the 

 standard should be taken rather as 3 than 4 volumes per 10,000. 



Although carbon dioxide does not itself support animal life, and we 

 could do very well without it in the atmosphere so far as breathing is 

 concerned, it is necessary to the growth of plants, and therefore through 

 them an indispensable substance for the existence of the human race. 

 The vegetable world not only needs a supply of this gas for its own 

 sustenance, but by the selective action of its leaves keeps the air con- 

 tinually pure enough for the life of animals. Under the influence of 

 sunlight every green plant absorbs the carbonic dioxide at its surface, 

 breaks it up into carbon and oxygen, and returns some free oxygen to 

 the atmosphere. In this way the two great kingdoms, the vegetable 

 and the animal, mutually contribute, each to the other, the elements 

 of life. The carbon drawn from the air, together with hydrogen and 

 oxygen, forms the wood of the tree, the stalk of the plant, and the 

 flesh of the fruit, and these, when burnt or eaten, again result in carbon 

 dioxide and water. 



The change from the compound gas to carbon and oxygen is brought 

 about by small openings or pores filled with a green substance, 

 chlorophyll, which during the daytime has the power to extract the 

 carbon and set free the oxygen. At night, on the contrary, there is a 

 slight expiration of carbonic dioxide, so that there is a real reason 

 against keeping large green plants in a bedroom during the night. 

 But the amount is very small compared with that exhaled by one 

 person. 



It is now known that plants, like animals, breathe oxygen from the 

 air, while they use the carbonic acid as food. 



About 1,346 cubic inches of carbonic dioxide are exhaled by a 

 healthy man per hour. An adult gives off in repose about 0.7 cubic 

 foot, and in active work about 1 cubic foot per hour. (Pettenkofer.) 



It is a remarkable fact that this amount is much reduced when the 

 air is already fouled with this gas; experiments showed that where 

 the same air was rebreathed, as it often is, the reduction was from 32 

 to 9.5 inches per minute. Thus it appears that the elimination of 

 waste products from the system is seriously checked by the presence 

 in the air breathed of an excess of carbonic dioxide. Otherwise 

 stated, air in crowded places may continue to sustain life while it fails 

 to remove any but a very inadequate portion of the poisons with which 

 the blood is charged. 



The general surface of the skin of the body also gives out a consid- 

 erable quantity of carbon dioxide, though, of course, very much less 

 than the lungs. 



