ttbe Btmospbere. 47 



amount of ammonia, perhaps not over one- 

 sixty-millionth. Oxygen also exists in the 

 air in very small quantities in another form 

 called ozone. One way to produce ozone 

 is by passing an electric spark through air. 

 Anyone who has operated a Holtz machine has 

 noticed a peculiar smell attending the disrup- 

 tive discharges, which is the odor of ozone. It 

 is what chemists call an allotropic form of 

 oxygen, just as the diamond, graphite, and 

 charcoal are all different forms of carbon, and 

 yet the chemical differences are scarcely trace- 

 able. It is more stimulating to breathe than 

 oxygen and is probably produced by lightning 

 discharges. 



As has been before stated, the oxygen of the 

 air is consumed by all processes of combustion, 

 and in this we include the breathing of men 

 and animals and the decay of vegetable mat-, 

 ter, as well as the more active combustion aris- 

 ing from fires. A grown person consumes 

 something over 400 gallons of oxygen per day, 

 and it is estimated that all the fires on the 

 earth consume in a century as much oxygen 

 as is contained in the air over an area of 

 seventy miles square. All of these processes 

 are throwing into the air carbon dioxide (car- 

 bonic acid), which, however, is offset by the 

 power of vegetation to absorb it, where the 

 carbon is retained and forms a part of the 

 woody fiber and pure oxygen is given back into 



