84 PEOCESSES OUTSIDE THE LIVING CELL CHAP. 4 



formaldehyde is formed under these conditions. Stoklasa and Zdobnicky (1912, 1913) 

 emphasized particularly the alleged photochemical production of formaldehyde from 

 carbon dioxide in presence of nascent hydrogen. They suggested that this is the actual 

 photochemical reaction in photosynthesis, and that nascent hydrogen can be formed in 

 the plant by an enzymatic reaction. Since the reduction of carbon dioxide by hydrogen 

 liberates a small amount of energy, while the dissociation of water into hydrogen and 

 oxygen requires even more energy than photosynthesis itself; 



(4.25a) 2 H2O > 2 H2 + O2 - 137 kcal 



(4.25b) 2 H2 + CO2 > {CH2OI + H2O + 25 kcal 



(4.25) H2O + CO2 > {CH2O) + O2 - 112 kcal 



— it is obviously absurd to suggest that the first reaction is thermal and the second 

 photochemical. 



3. Sensitized Reduction of Carbon Dioxide 



This section describes the investigations in which "artificial photo- 

 synthesis" has been claimed as an accomplished fact. Their usual 

 technique was to illuminate carbon dioxide solutions in presence of 

 different "sensitizers" and then search for traces of formaldehyde or 

 other organic compounds as ardently as alchemists have searched for a 

 grain of gold in the bottom of their crucibles. More often than not, 

 no specific reductant was provided for the reduction of carbon dioxide, 

 and the assumption that water acted as such was made without any 

 attempt to confirm it by proving the liberation of oxygen. 



In our discussion of these experiments, we will endeavor to keep 

 apart the two phenomena defined in the discussion of the sensitized oxi- 

 dation of water: true photocatalysis (either of the decomposition of carbon 

 dioxide, or of the reduction of carbon dioxide by a specific reductant) 

 and photoreduction of carbon dioxide (or its derivatives) by the "sensi- 

 tizer" itself. 



In 1893, Bach found that a solution of carbon dioxide and uranyl 

 acetate reacts in light; uranium oxides are precipitated, and Bach thought 

 that carbon dioxide might be reduced to formaldehyde. The same 

 "sensitizers" (uranyl salts) were used by Usher and Priestley (1906) 

 and Moore and Webster (1918). The last named authors thought the 

 colloidal state of the sensitizer to be of particular importance; they 

 obtained positive formaldehyde tests in illuminated carbonate solutions 

 containing colloidal uranium and iron salts, and thought that these 

 results afford an explanation of natural photosynthesis, since colloidal 

 iron compounds are present in the chloroplasts (c/. Chapter 14, page 376). 

 Apart from doubts concerning the correctness of the experimental results 

 of Moore and Webster (c/. page 89), we must ask what happened in 

 these experiments to the "sensitizers." Did they remain unchanged, 



