Nitrates in Aqueous Solution hy the Action of Sunlight. 163 



suspected of containing nitrites (8). This test is given only by nitrites, and 

 not by either ozone or hydrogen peroxide. 



Ilosvay (6) by the use of this test showed that the well-known reaction 

 upon a paper impregnated with starch and potassium iodide often used to 

 show the supposed presence of ozone in the atmosphere was really produced 

 by nitrites, and demonstrated that at the earth's surface both ozone and 

 hydrogen peroxide were normally absent. The same observer (5) found a 

 strong reaction for 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 three parts hy 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 concentrations 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 relation- 

 ship 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 atmo- 

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



