72 FOKMATION OF NITKITES FROM NITRATES 



nitrite. These figures show clearly that the transition from nitrate 

 to nitrite is a strongly endothermic reaction, and can only occur 

 either by transformation of other forms of energy, such as that of 

 light, into chemical energy, or by a linked chemical reaction with 

 oxidation of previously formed reduced chemical substances. 



Not only do the nitrites contain a greater storage of chemical 

 energy than the nitrates, the energy potential factor of the energy 

 quantity possesses a higher value, so that the nitrites react more 

 readily than the nitrates, and many changes occur between living 

 organisms and nitrites which are not given directly by nitrates. 

 The experiments recorded below show that this endothermic reaction 

 occurs in sunlight when dilute solutions of nitrates are exposed 

 to the direct rays of the sun, either dissolved in redistilled water or 

 as they occur in natural waters. In addition, it has been found that 

 green leaves immersed in water possess, in presence of sunlight, the 

 power of absorbing the nitrites so formed in the water. 



In the earlier stages of the investigation the source of the nitrites 

 obtained was not clearly understood. The outset point was that 

 the nitrites and nitrates found to be present in atmospheric air by 

 many previous observers could not be satisfactorily explained on 

 the basis of the disruptive electrical discharges of thunderstorms, 

 because there has not been traced any definite correspondence 

 between the amount of nitrites, and nitrates in air and rain water, 

 and the prevalence of thunderstorms, such as must undoubtedly 

 exist were the energy of the lightning discharge the main cause of 

 the production of nitrites and nitrates in the air. 



Rain collected in the course of these experiments when there had 

 been no recent thunder was found to contain nearly as much nitrite 

 (about 0-5 part per million) as rain caught in a thunderstorm, and, 

 as has been shown by Ilosvay, 1 the morning dew contains nitrites. 

 Also, although the amounts of nitrites and nitrates present at any 

 given time in the air are small, the amounts abstracted by condensing 

 aqueous vapour and falling as rain or condensing as dew on the sur- 

 faces of leaves and ground in the course of the year is enormous, and 

 this would appear to demand some uniformly distributed and more 

 constantly acting source of energy, such as sunlight, rather than be 

 dependent upon fortuitous electrical discharges. 



These nitrites of the rain and dew form one of the chief supplies 

 of nitrogenous nutrition for plants and animals supported by soils 

 1 Ilosvay, Bull. Soc. Chim. [3], vol. ii., p. 666. 



