Nitrates in Aqueous Solution by the Action of Sunlight. 16] 



this point, whether the starting point be nitrous oxide (N2O) or nitrogen 

 pentoxide (^205). The amounts of heat involved and differences in transition 

 from one oxide to another are shown succinctly in the following Table 

 abstracted from Mendeleeff (4), in which the numbers in the upper row 

 represent thousands of gramme-calories for a gramme-molecular formation 

 from the elements ; and the lower shows in thousands of gramme-calories 

 the heats of transition from one oxide to the other : — 



N2O N2O2 IT2O3 N2O4 N2O5 



-21 -43 -22 -5 -1 



-22 +21 +17 +4 



> 



This Table shows that in passage from ^205 to ^203, a supply of energy 

 must be given to the reacting system amounting to 4-f 17 = 21 thousands 

 of gramme-calories for each gramme-molecule converted. The figures are for 

 the gaseous condition, but it may be taken that they would be approxi- 

 mately the same for dilute solutions, and so that the amount of energy for 

 the passage of a gramme molecule of a nitrate to a nitrite (say, from KNO3 

 to KNO2) would be about half this amount or about 10,000 grm. cal., 

 for 1 grm. mol. of nitrogen pentoxide yields two of nitrate, which is 

 converted into two of 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. 



I«J"ot 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 



