68 HOW CROPS FEED. 



themselves with this subject. Tlie most recent investiga' 

 tions of Daubeny, {Journal Ghem. 8oc., 1867, pp. 1-28,) 

 lead to the conclusion that ozone is exhaled by plants, a 

 conclusion previously adopted by Scoutetten, Poey, De 

 Luca, and Kosraann, from less satisfactory data. Dau- 

 beny found that nir deprived of ozone by streaming 

 th.-ough a solution of iodide of potassium, then made to 

 pa-^s the foliage of a plant confined in a glass bell ami e^- 

 poseil to sunliglit, actjuired the power of blueing iodide 

 of-potassium-starch-paper, even when the latter was shield- 

 ed from the liglit.* Cloez, however, obtained the contrary 

 results in a series of experiments made by him in 1855, 

 {Ann. de Chimie et de Fhys., L, 3:26,) in which the oxy- 

 gen, exhaled both from aquatic aiul 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, {Phil. 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 Cloez's experiments, but 

 spent itself in some oxidizing action before it reached the 

 test-paper. In Daulieny's experiments, however, the more 

 rapid stream of air might have carried along over the test- 

 ]iaper 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. — Atinospheric electricity, 

 slow oxidation, and combustion, are obvious means of im- 

 pregnating the atmosjihere more or less with ozone. If 



* Li^lit alone blues this paper after a lime in absence of ozone. 



