304 PHOTOSYNTHESIS 



Baly, very small, two to eight parts per million, though this can be 

 increased if calcium or potassium bicarbonate is present. It is asserted 

 that when the solution of carbon dioxide is not agitated the formaldehyde 

 produced is so rapidly polymerized to carbohydrates that no tests for 

 formaldehyde can be obtained. If the solution is agitated some of the 

 formaldehyde is carried to the back of the vessel where it escapes polymeri- 

 zation. Baly reports having obtained tests with Benedict solution which 

 indicate the presence of about 0.005 per cent reducing sugar. In some 

 experiments Baly used various organic compounds to protect the formal- 

 dehyde "from those specific rays which cause polymerization." For this 

 purpose he used paraldehyde and sodium phenoxide as well as colored 

 compounds as malachite green and methyl orange. It is extremely doubt- 

 ful whether these substances when illuminated with ultra-violet light do 

 not themselves form formaldehyde or compounds which give reactions 

 which may be mistaken for formaldehyde. The same applies to the experi- 

 ments in which a solution containing sodium carbonate, sodium citrate 

 and copper sulfate (Benedict solution) was illuminated and reduction of 

 the copper observed.^^^ That cupro-alkaline solutions such as Fehling's 

 solution are decomposed by light has been known for a long time and 

 there is little reason to believe that the same does not apply to Benedict's 

 solution. 



There is unfortunately very little agreement in the results of various 

 experimenters on the reduction of carbon dioxide in ultra-violet light. 

 Berthelot and Gaudechon obtained formaldehyde only in the presence of 

 hydrogen, Stoklasa only when the hydrogen was in "statu mascendi," 

 Usher and Priestley obtained formaldehyde "in a polymerized form" 

 and Baly and his collaborators claim formaldehyde and sugar. With 

 these must be included the experiments in which only negative results 

 were obtained. 



Spoehr ^-'^ was unable to obtain any definite evidence of the forma- 

 tion of formaldehyde from carbon dioxide in several dififerent series of 

 experiments. Quartz mercury vapor lamps of several makes were used, 

 the temperature, pressure of carbon dioxide, distance from the source of 

 ultra-violet light and duration of illumination were all varied. Stoklasa's 

 experiments were repeated using hydrogen prepared electrolytically. No 

 definite test for formaldehyde was ever obtained. Small amounts of 

 formic acid were formed and when formic acid was illuminated with 

 ultra-violet light a small amount of a non-volatile sirup was obtained. 

 This reduced Fehling's and Tollen's solutions and algae developed in 

 solutions thereof in the dark ; it was also decomposed in ultra-violet light 

 so that only small amounts were obtained. Thiele ^^^ has observed a 

 similar action of ultra-violet light on formic acid. 



^'Plotnikow, "Lehrbuch der Photochemie," 1920, p. 330. 



"•Spoehr, Biochem. Zeit., 57, 110 (1913); Plant World, 19, 1 (1916); Totir. 

 Anier. Chem. Soc, 45, 1184 (1923). 



"^Thiele, Zeit. angew. Chem., 22, 2472 (1909). 



