THE PLANT AND THE ATMOSPHERE 49 



Schone, 1 is hydrogen peroxide, though it is often said to be ozone. 

 This substance is destroyed by putrescible substances, and it 

 destroys bacteria. The presence of hydrogen peroxide is thus 

 evidence of the purity of the air as regards freedom from bacteria 

 and putrescible bodies. Hydrogen peroxide does not occur in 

 the air of towns or marshes, since any formed is instantly 

 destroyed by the organic matter present. 



At Montsouris, near Paris, the amount of hydrogen peroxide 

 in the air was estimated to be on, an average, about one part in 

 100,000,000 for a period covering thirteen years. 



Hydrogen peroxide may be formed by electrical discharge 

 (lightning) and in some processes of oxidation. It acts upon 

 iodide of potassium, liberating iodine, which turns starch blue. 

 Paper impregnated with potassium iodide and starch is a delicate 

 test for ozone, or hydrogen peroxide, since very small quantities 

 of these substances suffice to turn it blue. 



Other Constituents of the Air. Marsh gas (CH 4 ) is a colorless 

 and odorless gas produced in the decay of vegetable matter under 

 water, as in marshes, and in the digestion of hay and other food 

 by herbivorous animals. Small quantities of it occur in the air. 



Sulphur dioxide may occur in the air in the neighborhood of 

 smelters, factories, or in towns. If present in appreciable quan- 

 tity, it is injurious to vegetation. It is evolved from the oxida- 

 tion of sulphur during the combustion of coal. 



Dust particles, organic matter, and salts from the evaporation 

 of the spray of the sea, are found in the air. 



Bacteria are also present, in much greater numbers in the city 

 than in the country. Levy found 345 per cubic meter in the air 

 of Montsouris, 4,790 in the air of Paris (average of 13 years). 



Composition of Rain Water. Rain water has been converted 

 into vapor by the sun, and condensed again into a liquid. In 

 its passage through the air, rain water takes up ammonia, nitrates 

 dust, chlorides and other constituents of the air. Rain water, 

 therefore, though it is the purest natural water, is not absolutely 

 pure. 



1 Berichte, 1880, p. 1503. 



