ATMOSPIiHUIC AIU AS THE FOOD OF PLANTS. 65 



Tests for Ozone.— Certain phenomena of oxidation tliat are 

 attended with changes of color serve for the recognition of ozone. 



We liave alreiuly seen (II. C. G., p. 64) that starch, when brought in 

 contact with iodine, at once assumes a dci-];- blue or purple color. When 

 the compound of iodine with potassium, known as iodide of potas- 

 sium, is 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 i\ 

 mi.xture of starch-paste and solution of iodide of potassium,* we have i^ 

 test of the pi'esence of ozone, at once most chaiaeteristie and delicate. 



Sucli ]>apcr, moi.-tcned and placed in ozonoust air, is spcidi^j turnei\ 

 blue by tiie action of the liberated iodine upon the starch. By the usq 

 of this test the presence and abundance of oz<.>nte in tiiti atmosphere h;i^ 

 been measured. 



Ozone is Active Oxygen. — That ozone is nothino; jnore 

 or less than o.vygen 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 sih'er in a state of fine division, it is possible by 

 long-continued transmissicn of electrical discharges to 

 cause the gaseous oxj^gen entirely to disappear. On heat- 

 ing the silver, which has boconie 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 e.^perimonts of Andrews, Babo, and 

 Soret, demonstrate that ozone has a greater density than 

 oxygen, since the latter diminislies 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. 



* Mis 10 parts of starch with 200 parts of cold water and 1 part of recently 

 fused iodide of potassiiuu, by nibbing them together in a 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. 



t I. c., charged with ozone. 



t Recent observations by Babo and Glaus, and by Soret, show that the density 

 tf ozone is one and a half times greater than that of oxygen. 



