ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 65 
Tests for Ozone.—Certain phénomena of oxidation that are 
attended with changes of color serve for the recognition of ozone. 
We havealready seen (H. C. G., p. 64) that starch, when brought in 
contact with iodine, at once assumes a ‘deep blue ar purple color. When 
the compound of iodine with potassium, known as iodide of potas- 
sium, Js acted on by ozone, its potassium is ‘at once oxidized (to pot- 
ash,) and the iodine is set free. If now paper be impregnated with a 
inixture of starch-pasté and solution of iodide of potassium,* we have a; 
test of the presence of ozone, at once most characteristic and delicare. 
Such paper, moistened -and placed in ozonous +} air, is speedily turned 
blue by the action of the liberated iodine upon the starch. By the use 
of this test the presence and abundance of ozune ix the atmosphere has 
been measured. 
’ Ozone is Active Oxygen.—That ozone is nothing more 
or less than oxygen in a peculiar, active condition, is shown 
by the following experiment. When perfectly pure and 
dry oxygen is enclosed in a glass tube containing moist 
metallic silver in a state of fine division, it is possible by 
long-continued transmission of electrical discharges to 
cause the gaseous oxygen entirely to disappear. On heat- 
ing the silver, which has become blackened (oxidized) by 
the process, the original quantity of oxygen is recovered 
in its ordinary state, The oxygen is thus converted under 
the influence of electricity into ozone, which unites with 
the silver and disappears in the solid combination. 
The independent experiments of Andrews, Babo, and 
Soret, demonstrate that ozone has a greater density than 
oxygen, since the latter diminishes in volume when elec- 
trized. Ozone is therefore condensed oxygen,{ i. e., its 
molecule contains more atoms than the molecule of ordi- 
nary oxygen gas. 
* Mix 10 parts of starch with 200 parts of cold water and 1 part of recently 
fused iodide of potassium, by rubbing them together ina mortar; then heat to 
boiling, and strain through linen. Smear pure filter paper with this paste, and dry 
The paper should be perfectly white, and must be preserved in a well-stoppered 
bottle. 
+ Le., charged with ozone. 
+ Recent observations by Babo and Claus, and by Soret, show that the density 
-f ozone is one and a half times greater than that of oxygen. 
