ATMOSPHERIC AIR AS THE POOD OF PLANTS. 69 



the oxygen exhaled by plants contams ozone, this sub- 

 stance must be perpetually formed in the atmosphere over 

 a large share of the earth's surface. 



The quantity present in the atmosphere at any one time 

 must be very small, since, from its strong tendency to unite 

 with and oxidize other substances, it shoitly disappears, 

 and under most circumstances cannot manifest its peculiar 

 properties, except as it is continually reproduced. The 

 ozone present in any part of tlie atmosphere at any given 

 moment is then, not what has been formed, but what re- 

 mains after oxidable matters have been oxidized. We find, 

 accordingly, that atmosplieric ozone is most abundant in 

 winter; since then there not only occurs the greatest 

 amount of electrical excitement * in the atmosphere, which 

 produces ozone, but the earth is covered with snow, and 

 thus the oxidable matters of its surface are prevented 

 from consuming the active oxygen. 



In the atmospliere of crowded cities, in the vicinity of 

 manure heaps, and wherever considerable quantities of or- 

 ganic matters pervade the air, as revealed by their odor, 

 there we find little or no ozone. There, however, it may 

 actually be produced in the largest quantity, though from 

 the excess of matters which at once combine with it, it 

 cannot become manifest. 



That the atmospliere ordinarily cannot contain more 

 than the minutest quantities of ozone, is evident, if we 

 accept the statement (of Schonbein ?) tliat it conmiunicates 

 its odor distinctly to a million times its weight of air. 

 The attempts to estimate the ozone of the atmosphere give 

 varying results, but indicate a proportion fir less than 

 sufficient to be recognized by the odor, viz., not more than 

 1 part of ozone in 13 to 65 million of air. (Zwenger, 

 Pless, and Pierre.) 



These figures convey no just idea of the quantities of 



* The amount of electi-ical disturbance is not measured by the number and 

 Tiolence of thunder-storms : these only indicate its intensity. 



