372 SCHAEFFER. 



air path, X2313 would be shown stronger than X2573. In no case did 

 this occur. Photographs taken for the five mile distance agree with 

 the work of Strutt in showing that a concentration of ozone in the air 

 near sea-level greater than 10"^ is improbable. Chemical determina- 

 tions give results over a thousand times this amount, but the difficul- 

 ties of such determinations are very great and the data given by 

 several observers differ widely. 



Evidently, the problem of the effect of our atmosphere on ultra- 

 violet light has been resolved into two parts. The first calls for an 

 explanation of the abrupt termination of the solar spectrum in the 

 ultra-violet, the second calls for an explanation of the deviation of the 

 intensity of a distant terrestrial source from the curve calculated from 

 a consideration of molecular scattering. 



Properties of Ozone. — The limit of the solar spectrum is certainly set 

 by ozone absorption. From the chemical and physical properties of 

 ozone, there is reason to believe that there is a fair concentration of 

 that gas probably in the isothermal layer of our atmosphere where the 

 mean temperature is about —55° C and the pressure about 250 m.m. 

 Ozone is quite stable at this low temperature and although the sta- 

 bility decreases with decreasing pressure the effect is not very large 

 until the pressure ^^ is beloAv 100 m.m. It is also important to note 

 here that ozone is produced quite rapidly by light of wave-length 

 less than X2000, but is slowly recombined by light of longer wave- 

 length. Accordingly, light of waA'e-length less than X2000 coming 

 from the sun, forms ozone in the higher layers of our atmosphere. 

 This ozone should be found in higher concentration in the isothermal 

 layer since the conditions of temperature and pressure are the most 

 favorable there. ^^ Such a layer of ozone would account for all the 

 observed facts on the absorption of sun-light in the region above X2000. 



The data collected by several observers on the ozone content at 

 varying altitudes ^'^ are in fair agreement in showing a gradual increase 

 up to 3600 meters. Pring ^^ gives average values obtained by sending 

 his apparatus up as high as 20,000 meters in Hydrogen balloons but 

 the results were hopelessly discordant. A greater percentage of the 

 heavier molecules as O3 should be found at the lower altitudes, but the 

 very rapid recombination of O3 into O2 at temperatures found near the 

 surface of the earth and the violent chemical action of ozone on or- 

 is von Bahr, Ann. d. Physik, 33, 3, p. 598, 1910. 



16 Warburg, Ber. Preuss. Akad. Wissens., Berlin, 48, p. 1126, 1901. 



17 Thierry, Comp. Rend., 124, p. 460, 1897. 



18 Pring, Proc. Roy. Soc, London, 90A, p. 204, 1914. 



