SCIENCE OF COMMON THINGS. 211 



* ,. 



Close rooms unhealthy. Carbon thrown off by the lungs. 



1405 Why are persons who live in close rooms and crowded' cities 

 generally sickly f 



Because the air they breathe is not pure, but is (in 

 the first place) defective in oxygen, and (in the second) 

 is impregnated with carbonic acid gas. 



1406 Where does the carbonic acid of close rooms and cities com* 

 from ? 



From the lungs of the inhabitants, the sewers, drains, 

 and other like places, in which organic substances are 

 undergoing decomposition. 



1407 What becomes of the carbonic acid generated in crowded cities f 



It is gradually diffused through the air, absorbed by 

 vegetation and by water, and wafted by the winds to 

 different localities. 



1408 Does not this constant diffusion of carbonic acid affect the purity 

 of the whole air 1 



No ; because it is wafted by the wind from place to 

 place, and absorbed in its passage by the vegetable world. 



1409 What is choke damp ? 



Carbonic acid gas accumulated at the bottom of wells 

 and pits, which renders them noxious, and often fatal to 

 life. It is called choke damp, because it chokes (or suf- 

 focates) every animal that attempts to inhale it. 



It suffocates without getting into the lungs, by closing the glottis spas' 

 modicatty. 



1410 Why is not this carbonic acid taken up by the air and diffused, as 

 it is in tities f 



Because (being heavier than common air) it cannot 

 readily rise from the well or pit ; and no wind can get 

 to it to blow it away. 



By the chemical law of diffusion, a portion of the carbonic acid which 

 accumulates at the bottom of wells and pits, is removed ; but in many 

 cases this abstraction is more than counterbalanced by an increased 

 supply. 



1411 How much carbon in the form of carbonic acid passes through the 

 lungs of a healthy person every twenty-four hours f 



The quantity would be very accurately represented 

 by a mass of charcoal of the weight of fifteen ounces. 



The volume of carbon in the atmosphere, although it forms but one per 

 cent, of the carbonic acid existing in it, exceeds in amount all the carbon 



