70 AGRICULTURAL CHEMISTRY 



rather because it modifies the conditions of the at- 

 mosphere than because it furnishes moisture directly for 

 plant growth. The humidity of the air also influences 

 many farm operations, as the curing of cheese, which is 

 best effected in an atmosphere containing about 85 per 

 cent, of moisture. 



71. Ozone and Hydrogen Peroxid. These are oxidi- 

 zing agents, present in the air only in traces. Ozone is 

 a modified or allotropic form of oxygen. It is more 

 active than ordinary oxygen. Hydrogen peroxid (H 2 O 2 ) 

 readily gives up one of its atoms of oxygen for oxidation 

 purposes. H 2 O 2 = H 2 O -f O. 



72. Argon and Helium are elements which are present 

 in the air in small amounts. Argon makes up about i 

 per cent, of the volume of the air, and is like nitro- 

 gen in many of its chemical characteristics, but is even 

 more inactive and inert than nitrogen ; it is the most in- 

 active element known. Argon is not supposed to take 

 any direct part in animal or plant life. 



73' Organic Impurities. The amount of dust, dirt, 

 and impurities in the air varies with the conditions, as 

 rainfall, local influences, and sources of contamination. 

 Fine particles of dust, containing decaying vegetable mat- 

 ter, are carried long distances by the wind. This decay- 

 ing vegetable matter often contains spores of disease 

 germs. The dust and impurities in the air can be ob- 

 served when a beam of sunlight finds its way into a room; 

 then the particles of dust will be seen floating in the air. 

 When the air in a room is not in motion, the dust parti- 



