SENSITIZED REDUCTION OF CARBON DIOXIDE 89 



light. Once in a million absorption acts two photons may strike the 

 same molecule or two excited molecules may collide and exchange 

 energy, accumulating a quantum sufficiently large to cause the formation 

 of a free atom or radical. Accidents of this kind may lead to the 

 formation of a few molecules of formaldehyde in carbonate solutions 

 subjected to a prolonged irradiation by visible light. The essential 

 characteristic of natural photosynthesis is that the accumulation of 

 energy occurs w4th an efficiency far in excess of anything explicable by 

 statistical considerations. Unless we are able to imitate nature in this 

 respect, we have no right to speak of having achieved "artificial photo- 

 synthesis" — even if we should succeed in producing traces of formalde- 

 hyde by a prolonged illumination of carbonate solutions. 



(b) The Experiments of Baur 



The series of papers by Baur and coworkers, dealing with artificial 

 photosynthesis and related processes in a variety of systems in vitro, 

 remain to be discussed. In many respects, they compare advantageously 

 with the attempts of Baly and Dhar. Unfortunately, Baur's adherence 

 to a strange theory — the reduction of all photochemistry to electro- 

 chemistry {cf. page 90) makes the reading of his papers difficult. The 

 variety of systems investigated by Baur and coworkers was imposing, 

 and the results were always reported in a scrupulous fashion. Neverthe- 

 less, we do not believe that artificial photosynthesis has been achieved 

 by Baur. Aside from the one very complex system (acetate silk- 

 chlorophyll-cetyl alcohol) whose illumination allegedly yielded as much 

 as 20 moles of formaldehyde per mole of chlorophyll present, the essence 

 of all the other experiments was the formation of formaldehyde in 

 quantities roughly equivalent to those of the sensitizing dyes used, and 

 very small compared with the total quantity of the other organic compo- 

 nents of the reacting system. The assumption that this formaldehyde 

 was formed by the reduction of a carboxyl group, or of carbonic acid 

 (and not by oxidation of an alcohol or hydrocarbon) cannot be considered 

 as proved. The formation of oxygen was claimed only in an experiment 

 which was termed by Baur himself as "prefiminary" and in a recent in- 

 vestigation, of which only an abstract could be obtained (c/. p. 93). 



ffis first papers (Schiller and Baur 1912, Baur and Rebmann 1922, and Baur and 

 Buchi 1923) were concerned with the refutation of the claims by Usher and Priestley 

 (1906), Moore and Webster (1913, 1918) and Baly, Heilbron and Barker (1921). In 

 addition to showing that colloidal ferric oxide, ferric chloride, uranium oxide, sodium 

 uranate, and malachite green do not convert carbon dioxide in hght into formaldehyde 

 or formic acid (as asserted by the above-mentioned authors), Baur and Biichi (1923) 

 also investigated the action of dyestufifs (eosin, phosphine, malachite green) in non- 

 aqueous systems (lecithin emulsions in xylene), as well as in the adsorbed state (on 

 cotton and silk fiber) in the form of resinates, etc. No oxygen evolution was observed, 



