BY THE ACTION OF SUNLIGHT 77 



nitrogen fixation from the atmosphere. The light of the sun in the 

 upper strata of the atmosphere, where the ultra-violet has not yet 

 been absorbed, must produce vast amounts of ozone, and these dis- 

 appear as the earth's surface is approached; in so disappearing, the 

 ozone may oxidise nitrogen and give nitrites and nitrates. What- 

 ever their source, these nitrites are most reactive with organic 

 substances and destructive to micro-organisms, and many of the 

 natural bleaching, deodorising, and sterilising activities in air 

 and water which have been hitherto ascribed to ozone and hydrogen 

 peroxide are more probably due to nitrites. 



SUMMARY. 



Dilute solutions of nitrates exposed either to sunlight or to a 

 source of light rich in light energy of short wave-length (such as 

 light from mercury vapour arc enclosed in silica) undergo conversion 

 of nitrate into nitrite. 



There is an uptake of chemical energy in this reaction transformed 

 from light energy as in formation of organic carbon compounds in 

 foliage leaves; it is to be added to the relatively small number of 

 endothermic reactions known to be induced by light. 



Interposition of a layer of glass between source of light and solu- 

 tion of nitrate greatly slows the reaction, showing that the most 

 effective rays are those of short length. 



When green leaves are immersed in nitrate solution, compara- 

 tively little nitrite accumulates, indicating that nitrites are rapidly 

 absorbed by the green leaf. Nitrates taken up by plants from soil 

 would, in presence of sunlight, be changed to nitrites, which are 

 much more reactive than nitrates. This indicates that the early 

 stages of synthesis of nitrogenous compounds are carried out in the 

 green leaf and aided by sunlight. 



Rain water collected for a considerable time contains no nitrites, 

 all having been oxidised to nitrates, but if exposed to bright sunlight 

 or ultra-violet light for a few hours a strong reaction for nitrites is 

 always obtained. 



Freshly collected rain water or dew always contains a mixture 

 of nitrites and nitrates, as shown by the nitrite test appearing 

 without any previous treatment of the water and the great enhance- 

 ment of this on exposure to strong illumination. 



Air bubbled through nitrite- and nitrate-free distilled water 



