292 



SMITH'S INTERMEDIATE CHEMISTRY 



hour. In the lungs some of the oxygen is removed, and some 

 carbon dioxide is added. 



A candle flame goes out when the proportion of oxygen has fallen 

 to 16.5 per cent. But air will sustain life until the proportion has 

 fallen to about 10 per cent. 



Nearly all experts are now convinced that the unhealthiness 

 of over-crowded, " stuffy " rooms is not due to the increase in the 

 proportion of carbon dioxide, which is seldom great enough to do 

 any damage. Nor is it due to " poisons " given off by the lungs or 

 skin. In spite of many experiments the presence of such sub- 

 stances has never been proved they are imaginary. The 

 harm is caused by the stillness of the air, which, as we have seen 

 (p. 291), prevents the removal of the water vapor near the skin, 

 and therefore hinders evaporation. 



Dust in the Air. A beam of sunlight crossing a dark room 

 can be seen by the light reflected from the particles of dust which 

 all air contains. These are chiefly solid bodies, and are composed 

 of salts, limestone, clay, and other rock materials, of soot and other 

 particles of unburnt fuel, of bits of hay or straw, and of fragments 

 of insects and other debris of plants and animals. They also 

 include living particles, such as bacteria, and spores of plants 

 such as moulds. The latter, when they settle upon food, germi- 

 nate and give rise to putrefaction. Some of the bacteria also pro- 

 duce disease, when they enter the body at a place where the skin 

 has been damaged by a cut or burn. 



It is instructive to note that natural soil contains about 100,000 

 micro-organisms, and good, unfiltered river water from 6000 to 



