INSENSIBLE MOVEMENTS IN AIR. 325 



most important office in nature's chemistries. 

 Powerful as is the influence of the diffusive 

 force of gases in dispersing abroad the noxious 

 emanations of any particular district or region, 

 it may be reasonably doubted whether this 

 force is anything like sufficient of itself to pre- 

 serve the purity of a densely-populated region. 

 It is true we might not be sensible of any ill 

 effects from even a three days' calm in our own 

 temperate climate, although such a state of the 

 air seldom endures for more than a few hours. 

 But this could not long continue without 

 originating very serious evils. If we imagine 

 a crowd of human beings placed in a hall, how- 

 ever great its magnitude, and the air of such 

 a building entirely without motion, it is easy 

 to foresee the result. The impure emanations 

 from the lungs and bodies of so many human 

 beings would accumulate in this motionless mass 

 of air to such an extent as to render it in a lesser 

 or greater period of time altogether irrespirable, 

 and death would be the consequence of con- 

 tinuing under its influence. 



Such also would be the condition of a great 

 city over whose hundreds of thousands of inha- 

 bitants hung an atmosphere totally without 

 motion, without a breeze to fan the cheek or a 

 storm to intermix the varioiis parts of air and 

 impurities together. Under such circumstances 



