82 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



the recombination of carbon monoxide and oxygen. The photostation- 

 ary state: 



light 



(4.24) COo . CO + ^ O2 



light 



was investigated by Coehn and Sieper (1916), Tramm (1923), Coehn and 

 Spitta (1930) and Coehn and Maj^ (1934), with particular emphasis on 

 the effect of moisture. They found that the decomposition has a maxi- 

 mum at a certain moderate degree of drying, and is inhibited both by 

 an excess and by a complete absence of water. The mechanism of this 

 twofold effect of water remains unexplained. 



From the point of view of photosynthesis, it would be interesting if 

 this effect were due to a photochemical reaction between carbon dioxide 

 and water; but Thiele (1908) and Coehn and Sieper (1916) have asserted 

 that this is not the case, and that the influence of water is a purely 

 catalytic one. Also, Stoklasa and Zdobnicky (1910, 1911) and Stoklasa, 

 Sebor and Zdobnicky (1912, 1913) found that no formaldehyde is formed 

 by the illumination of carbonic acid in solution, while Baly, Heilbron 

 and Barker (1921), Dhar and Sanyal (1925) and Mezzadroli and Gardano 

 (1927) asserted that small quantities of formaldehyde can be obtained 

 in this way. This was disputed by Baur and Rebmann (1922), Spoehr 

 (1923) and Porter and Ramsperger (1925), who found the yield of 

 formaldehyde to decrease with increasing purity of the reactants. How- 

 ever, Baly, Davies, Johnson and Shanassy (1927) reiterated this organic 

 matter is formed by illumination of carbonate solutions with ultraviolet 

 light, claiming this time that it was not formaldehyde, but an undefined 

 higher aldehyde. Mezzadroh and Vareton (1931) claimed that the yield 

 of formaldehyde can be increased by preliminary ionization of carbon 

 dioxide by emanation or electric discharges. 



It is difficult to judge whether the newer claims of Baly, Dhar, and Mezzadroli and 

 coworkers deserve more confidence than the older ones. The positive test for formalde- 

 hyde in illuminated carbon dioxide solutions, described by Joo and Wingard (1933), 

 is anything but convincing, since it was obtained by Allison's "magneto-optic analysis," 

 whose reliability is open to serious doubts. 



If it should be confirmed that traces of formaldehyde are formed by 

 ultraviolet illumination of carbonate solutions, this phenomenon may be 

 of some interest from the point of view of the origin of organic matter 

 on earth. The first carbohydrates may have been formed, from carbonic 

 acid and water, by the ultraviolet light, which reaches the higher layers of 

 the atmosphere. Dhar and Ram (1933) have analyzed rain water and 

 found (by iodine titration) 1.5 X 10"^ to 1 X 10"^% of formaldehyde, 

 the larger values being obtained after long periods of sunshine. They 

 suggested that the photochenaical formation of formaldehyde occurs at, 



