THE CHEMISTRY OF PHOTOSYNTHESIS 305 



Coehn and Sieper ^-^ also studied the effect of ultra-violet light on 

 carbon dioxide under different conditions. While many photochemical 

 reactions are apparently sensitized by the presence of small amounts of 

 water this is not the case with carbon dioxide. The decomposition of 

 carbon dioxide is very greatly reduced by the merest trace of water 

 vapor. Mixtures of carbon dioxide and water yielded no formaldehyde. 

 Bauer and Rebmann ^^^ and Baur and Biichi ^^° repeated the experiments 

 of Baly, Heilbron and Barker, but were unable to obtain positive results 

 of the formation of formaldehyde from carbon dioxide and water. They 

 showed that formaldehyde is formed when various dyestuffs such as 

 malachite green, eosine, rhodamine. and phosphine are exposed to ultra- 

 violet light. They were also unable to obtain formaldehyde from car- 

 bon dioxide in the presence of colloidal ferric hydroxide or uranium oxide. 



Porter and Ramsperger ^^^ have made a very careful reinvestigation 

 of the action of ultra-violet light on carbon dioxide and water. They 

 found that "in every run where the illuminated gas came in contact with 

 rubber tubing, sealing wax, de Kootinsky cement or stop-cock grease, 

 small quantities of formaldehyde were obtained, but the results were never 

 quantitatively reproducible." When the entire apparatus was constructed 

 of quartz-to-glass seals, and conductivity water and carbon dioxide made 

 from pure sodium bicarbonate were used, after 36 to 60 hours' illumina- 

 tion, not a trace of formaldehyde or reducing sugars was obtained. The 

 failure to obtain positive results cannot be attributed to the fact that 

 the transparency of the quartz for light of short wave length had been 

 impaired by prolonged exposure to this light, for the same negative re- 

 sults were obtained with new quartz apparatus, and spectrographs showed 

 transmission down to 210 \i\i. 



It is difficult to understand to what the discrepancy in the results 

 of the action of ultra-violet light on carbon dioxide and water can be 

 attributed. The tests for very small quantities of formaldehyde must be 

 carried out with the greatest care in order to avoid spurious results. 

 It is possible that the positive results are to be attributed to dust 

 particles or other impurities as the results of Porter and Ramsperger 

 suggest. Or in those cases in which it is claimed that formaldehyde 

 is formed there existed conditions favorable to the formation of formal- 

 dehyde which were unknown to the experimenters. Porter and Ramsper- 

 ger conclude: "These experiments compel us to believe that if formalde- 

 hyde is formed from carbon dioxide and water alone, under the influence 

 of ultra-violet radiation, the reaction is reversible and the ordinary mer- 

 cury arc provides the energy required to decompose formaldehyde as 

 fast as it is formed. The question must be settled by determining whether 

 or not there is a definite region in the ultra-violet spectrum that will 



^ Coehn and Sieper, Zeit. physik-chem., 91, 347 (1916). Chapman, Chatwick 

 and Ramsbottom, Jour. Chem. Soc, 91, 942 (1907) ; 89, 22 (1908). 

 ^ Bauer and Rebmann, Hdv. chim. Acta. 5, 828 (1922). 

 ^'' Bauer and BGchi, ibid., 6, 959 (1923). 

 '"Porter and Ramsperger, Jour. Amcr. Chem. Soc, 47, 79 (1925). 



