CHAPTER V 



THE FORMATION OF NITRITES FROM NITRATES IN AQUEOUS 

 SOLUTION BY THE ACTION OF SUNLIGHT, AND THE 

 ASSIMILATION OF THE NITRITES BY GREEN LEAVES IN 

 SUNLIGHT 



THE number of chemical changes brought about by the activity of 

 light is multitudinous, and the study of these reactions has been very 

 intensive in recent years. In the majority of the photo-chemical 

 reactions, the effect produced is that of hastening an exothermic 

 reaction, and in this resembles the action of a catalyst. The sub- 

 stances formed have a less content of chemical energy than the 

 mother substances, and are usually of a more simple structural type. 

 In such cases there is no clear proof of transformation, or conver- 

 sion, of light energy into chemical energy, and the light acts more 

 as a detonator to a chemical reaction in which chemical energy is 

 set free. 



The most important case of an endothermic reaction set up by 

 the action of light is that in which the synthesis of formaldehyde 

 and carbohydrate is effected in the green leaf by that action of light 

 upon water and carbon dioxide in which the light energy is converted 

 into chemical energy and stored up. 



It has been shown in previous chapters that the first step 

 namely, that of formaldehyde formation in which the greatest up- 

 building of molecules with large storage of chemical energy occurs, 

 can be effected by certain catalysts, such as the ferric and uranium 

 salts in colloidal solution in water, when these are supplied with 

 light energy, and evidence has also been adduced that such in- 

 organic catalysts are present in the chloroplasts of green cells. 



If it be agreed that life at some period first arose on this or some 

 other planet by a process of evolution from simpler constituents, it 

 becomes of great importance to study fully the action of light upon 

 those inorganic substances which are present in air and water, and 

 might be presumed, from their nature and present position as 

 nutrients of living organisms, to have been capable of being acted 



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