298 BIOLOGICAL EFFECTS OF RADIATION 



checks. Furthermore, it has already been used in many researches as an 

 actinometer, and it is generally accepted as perhaps the best standard. 

 Although accurate only for monochromatic light, this reacting system can 

 be used with certain definite limitations as an actinometer for poly- 

 chromatic light (34). 



Ozone. — Oxygen is partially converted into ozone by ultra-violet 

 light of short wave-length— below 2000 A. The absorption spectrum of 

 oxygen in this region is discontinuous, and it is likely that the absorption 

 of a photon produces an activated molecule, which then collides with a 

 second molecule, producing two atoms of oxygen and one molecule of 

 oxygen. Each of these oxygen atoms can then combine with an oxygen 

 molecule, resulting in the formation of two molecules of ozone. If this 

 is the process, the corresponding equations would be: 



O2 + /iv = O2* 

 O2* + O2 = O2 + + O 

 + 02 = 03 

 and two molecules of ozone should be produced for each quantum 

 absorbed. Working at high pressures, 50 to 300 atmospheres, War- 

 burg (52) found a quantum yield approaching 2 but ranging from 2.06 to 

 1.3, depending on the pressure. Although the oxygen is much less 

 absorbing at the longer wave-lengths, there is still some production of 

 ozone by ultra-violet light at 2536 A. Short ultra-violet light from the 

 sun's rays is absorbed by the air long before it reaches the surface of the 

 earth and practically no ozone is produced directly from sunlight at sea 

 level. In the stratosphere, however, considerable amounts of ozone are 

 found. 



Ozone itself absorbs hght between 2300 and 2800 A and also in the 

 red end of the visible spectrum. It is decomposed by the ultra-violet 

 light which it absorbs with a quantum yield of 3.5 in pure ozone at 

 15°C. (47). 



When oxygen is radiated with polychromatic light containing both 

 the short ultra-violet and the longer ultra-violet, an equilibrium is set up, 

 ozone being formed and decomposed. A steady state is soon reached at 

 which the ozone is decomposed as fast as it is formed. The concentration 

 of ozone at this steady state depends on the relative intensities of the 

 longer and shorter ultra-violet. Quite frequently the smell of ozone is 

 detected around a freshly started mercury-vapor lamp but after the lamp 

 becomes heated the ozone is largely destroyed. 



Ozone can be decomposed also by light that it does not absorb, 

 provided a suitable photosensitizer is added. Light of 4360 and 4060 A 

 passed into a mixture of chlorine and ozone decomposes two molecules of 

 ozone for each quantum absorbed. This photochemical yield is prac- 

 tically independent of the ozone concentration over a tenfold range from 

 0.5 to 5 per cent and it is only slightly affected by temperature, but the 



