250 TiMEHRI. 



to quantity, but bear a dire6l but unestimated relation 

 to the density ot the population. 



Excepting the last mentioned, the list given includes all 

 the usual constituents of pure air, and as long as we only 

 breathe this mixture, we shall avoid a considerable number 

 of diseases and shall be under proper and natural condi- 

 tions in regard to our first food. And this is done by most 

 animals ; but immediately a tribe is formed and human 

 beings begin to be massed together in villages and 

 towns, immediately civilization requires buildings, con- 

 strufted, as they must be at first, without any scientific 

 knowledge of what is meant by pure air, or how it is to 

 be obtained ; then organic impurities of all descriptions 

 are added in various proportions to the air, and not 

 being diluted and destroyed rapidly enough, produce 

 disease and death. 



The standard composition of the air as given above 

 is principally maintained by what are known as the laws 

 of diffusion of gases, influenced by the temperature 

 and winds which produce a perfe6l mixture. The trade- 

 winds which affe6l us here, are usually said to be 

 due to the movements of the earth on its own axis, 

 together with the air at the equator being hotter than at 

 the poles, and so constantly rising, a vacuum is formed 

 into which cold air rushes. To show how little variation 

 there is in the principal gases of the air, I need only 

 quote from SMITH'S work " On Air and Rain," who gives 

 the oxygen in usual air as 20*96, in mountain air as 

 20*98 and in towns as from 20*90 to 20*87. ^" ^ 

 crowded street of Manchester, he found from 20*868 to 

 20" 1 79 per cent, of oxygen. In open spaces in London 

 20"95 per cent, was found, and in the crowded East End 



