74 FORMATION OF NITRITES FROM NITRATES 



nitrites in the morning dew on various leaves and grasses, and also 

 adsorbed upon ignited sand and upon earth exposed wet to the 

 atmosphere, and in water in absorption tubes through which air 

 was drawn. 



This test will clearly indicate, by the development of a pink 

 colour, the presence of nitrites in a dilution of one in ten million. 

 The test, when applied to the solutions exposed to sunlight, as 

 described below, gave reactions indicating amounts of nitrite lying 

 between two in a million and one in ten million. 



These concentrations may appear at first sight infinitesimally 

 low, but attention must be paid to the enormous areas in green leaves 

 over the earth's surface which are exposed to the reaction. The 

 strengths of solutions from which living organisms absorb essential 

 constituents from their environments often belong to this order of 

 concentration. The concentration of silicic acid in pond water, 

 from which diatoms build up their siliceous skeletons, is of the same 

 order of magnitude. A similar condition of affairs emerges if the 

 assimilation of carbon compounds is considered, for all such assimila- 

 tion depends on a concentration of only about 3 parts by volume 

 of carbon dioxide in 10,000 of atmospheric air. 



The concentration of carbon dioxide in the atmosphere of 

 3 parts in 10,000 by volume, small as it may appear to support all 

 life upon the earth, looks at first sight enormous, compared to the 

 concentrations at which silica is absorbed in plants, or to the con- 

 centrations of nitrites with which we are dealing in the present 

 experiments ; but this arises entirely from the usual conventional 

 mode of expression of the concentration in relative gaseous volumes 

 in the atmosphere. 



If the mass of carbon dioxide dissolved in water be expressed in 

 relationship to the mass of water, the ratio drops to the same order 

 of magnitude as obtains in the case of other essential constituents 

 demanded for organic life, all of which, it must be remembered, 

 including carbon, are synthesised from solutions and not in gaseous 

 form. Thus, the absorption coefficient of carbon dioxide between 

 a system of air and water may be taken sufficiently accurately for 

 these purposes as equivalent to unity, so that if an atmosphere 

 containing 3 parts in 10,000 of carbon dioxide be brought into 

 equilibrium with water, the aqueous solution will contain 3 

 volumes of gaseous carbon dioxide in 10,000 volumes of water. 

 That is, in 10 litres of water there will be dissolved 3 c.c. of carbon 



