Nitrates in Aqueous Solution by the Action of Sunlight. 165 



aud not conversion of dissolved nitrogen and oxygen, is shown by the fact 

 that it does not occur with distilled water holding air in solution ; but if a 

 small amount of potassium nitrate, say, one part by weight in 10,000 parts, 

 be added, an intense reaction occurs on insolation. In several experiments it 

 was shown that the presence of green leaves from different types of plants 

 diminished the amount of nitrites present after exposure, as contrasted with 

 control flasks alongside containing no green leaves. 



It is clear from these experiments that nitrates taken up by the rootlets 

 of plants from the soil can be converted into the more reactive nitrites in the 

 green leaf with absorption of solar energy ; simultaneous]}^, non-nitrogenous 

 organic bodies are being built up in the same situation, which suggests that 

 by interaction under the influence of light protein synthesis as well as carbo- 

 hydrate may occur in the green leaf. 



The presence of nitrites and nitrates in rain and dew indicates their 

 occurrence in atmospheric air, and this was ultimately proven in a series of 

 experiments which showed that the main portion of the oxidised nitrogen 

 from air is found in water, after bubbling air through it, as nitrate and not as 

 nitrite. 



Great care is required in order to give a rigorous proof of this, because the 

 condition of the absorbed substances froln the air may be modified in the act 

 of collection if light be not carefully excluded, and nitrate from the air be 

 changed by insolation into nitrite. This fact first emerged from a series of 

 experiments intended to study the relative amounts of nitrite in air by day 

 and by night, when apparently the interesting result was obtained that there 

 was practically no nitrite in night air, but a considerable amount in day air. 

 Just then the effect of light in converting nitrates into nitrites was learnt, 

 and a repetition of the experiment was made, using a blackened bottle with 

 distilled water as absorbent. This distilled water had been twice distilled, 

 and was so free from nitrate that it gave no Griess-llosvay reaction even 

 after prolonged exposure to ultra-violet light. The result now obtained was 

 that, whether the air were bubbled through by day or by night, only a very 

 slight reaction for nitrites was obtained ; but, on now exposing to sunlight 

 this distilled water through which air had been bubbled in darkness, whether 

 by day or by night, a strong reaction was obtained in each case, showing that 

 oxidised nitrogen is present always in air both by day and by night. 



It is not possible to conclude that the relative amounts of nitrate and 

 nitrite in bubbled air give an indication of the relative amoimts of the two 

 oxides of nitrogen in the .air ; for if the absorption be attempted in the 

 presence of light there will be a reduction to nitrite, and if in darkness, the 

 great volume of oxygen simultaneously bubbled through may have oxidised 



