ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 69 
the oxygen exhaled by plants contains 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 atmosphvre at any one time. 
must be very small, since, from its strong tendency to unite: 
with and oxidize other substances, it shortly disappears,. 
and under most; circumstances cannot manifest its peculiar 
properties, except as it is continually. reproduced. The, 
ozone ‘present in any part of the 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 atmospheric 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 atmosphere 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 atmosphere ordinarily cannot contain more 
than. the minutest quantities of ozone, is: evident, if we - 
accept the.statement (of Schénbein ?).that it. communicates. 
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 fur 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 electrical, disturbance is not siieanaeh by. the number and 
violence:of thunder-storms: these only-indicate its intensity; = 
