68 HOW CROPS FEED. 
themselves with this subject. The most recent investiga- 
tions of Daubeny, (Journal Chem. Soc., 1867, pp. 1-28,) 
lead to the conclusion that ozone is exhaled .by plants, a 
conclusion previously adopted by Scoutetten, Poey, De 
Luca, and Kosmann, from less satisfactory data. Dau- 
beny found that air deprived of ozone by streaming 
through a solution of iodide of potassium, then made to 
pass the foliage of a plant confined in a glass bell and ex- 
posed to sunlight, acquired the power of blueing iodide 
of-potassium-starch-paper, even when the latter was shield- 
ed from the liglt.* Cloéz, however, obtained. the contrary 
results in a series of experiments made by him in 1855, 
(Ann. de Chimie et de Phys., L, 326,) in which the oxy- 
gen, exhaled both from aquatic and land plants, contained 
in a large glass vessel, came into contact with iodide-of- 
potassium-starch-paper, situated in a narrow and blackened 
glass tube. Lawes, Gilbert, and Pugh, in their researches 
on the sources of the nitrogen of vegetation, (Ail. Trans., 
1861) examined the oxygen evolved from vegetable matter 
under the influence of strong light, without finding evidence 
of ozone. It is not impossible that ozone was really pro- 
duced in the circumstances of Cloéz’s experiments, but 
spent itself in some oxidizing action before it reached the 
test-paper. In Daubeny’s experiments, however, the more 
rapid stream of air might have carried along over the test- 
paper enough ozone to give evidence of its presence, Al- 
though the question can hardly be considered settled, the 
evidence leads to the belief that vegetation itself is a 
source of ozone, and that this substance is exhaled, to- 
‘gether with ordinary oxygen, from the foliage, when acted 
on by sunlight. 
Ozone in the Atmosphere. — Atmospheric electricity, 
slow oxidation, and combustion, are obvious means of im- 
pregnating the atmosphere more or less with ozone. If 
* Light alone blues this paper after a time in absence of ozone, 
