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 atmosphere 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 thie 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 communica‘es 
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 far 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 measured by the number and 
violence of thunder-storms: these only indicate its intensity. 
