BY THE ACTION OF SUNLIGHT 71 



energy, just as in the cases of the organic carbon compounds, was 

 the sun's rays. It follows that the agencies by which sunlight was 

 utilised to form reduced compounds of carbon and nitrogen must 

 have existed antecedently to the advent of life, for in its ultimate 

 composition the substratum of proteins necessary to the living organ- 

 ism contains both types of endothermically produced radicles. 

 It was such considerations which induced the series of experiments 

 here recorded, which show that the energy of sunlight can be 

 absorbed by dilute solutions of nitrates and institute an endothermic 

 reaction in which the more reactive nitrites are formed even in 

 absence of living organisms, and also that the green cells of plants 

 possess the power of absorbing these nitrites. 



It is well known from the thermo-chemical determinations of 

 Faure, Thomsen, and Berthelot that in the formation of the oxides 

 of nitrogen from their elements the acme of absorption of energy lies 

 at the point of formation of nitric oxide (N 2 2 ), and that the reac- 

 tion runs endothermically towards this point, whether the starting- 

 point be nitrous oxide (N 2 0) or nitrogen pentoxide (N 2 5 ). The 

 amounts of heat involved and differences in transition from one 

 oxide to another are shown succinctly in the following table 

 abstracted from Mendeleefi 1 in which the numbers in the upper 

 row represent thousands of grm. calories for a grm. molecular 

 formation from the elements ; and the lower shows in thousands of 

 grm. calories the heats of transition from one oxide to the other: 



N 2 N 2 2 N 2 3 N 9 4 N 9 5 



-21 -43 -22 -5 - - 1 



-22 +21 +17 +4 



This table shows that in passage from N 2 5 to N 2 3 a supply of 

 energy must be given to the reacting system amounting to 4+ 17=21 

 thousands of grm. calories for each grm. molecule converted. 

 The figures are for the gaseous condition, but it may be taken that 

 they would be approximately the same for dilute solutions, and so 

 that the amount of energy for the passage of a grm. molecule of 

 a nitrate to a nitrite (say, from KN0 3 to KN0 2 ) would be about 

 half this amount, or about 10,000 grm. calories, for 1 grm. molecule 

 of nitrogen pentoxide yields 2 of nitrate, which is converted into 2 of 



1 Mendeleeff, " Principles of Chemistry," translated by Kemensky and 

 Pope. vol. i., p. 287. Longmans. London, 1905. 



