38 THE SYNTHESIS OF CARBOHYDRATES 



light from a mercury-vapour lamp. The products of decom- 

 position are carbon monoxide, carbon dioxide, methane and 

 hydrogen ; aldehydic sugars differ from ketonic sugars both in 

 the readiness with which they are decomposed and in the 

 composition of the gaseous mixtures produced. 



Usher and Priestley * found that ultra-violet light can 

 bring about the decomposition of aqueous carbon dioxide with- 

 out the intervention of an optical or chemical sensitizer, a result 

 contrary to that of Stoklasa and Zdobnicky, who found that 

 formaldehyde was not produced by the action of ultra-violet 

 light on carbon dioxide and water. In view of these contrary 

 results, Spoehr f tried the effect of ultra-violet radiations on 

 carbonic acid and its salts ; in no experiment was a sufficiency 

 of formaldehyde produced to give a positive reaction with the 

 reagents employed. In all experiments Spoehr found that 

 formic acid was the only reduction product. Attempts to 

 reduce formic acid to formaldehyde by sun or by ultra-violet 

 light failed, but after ten to fifty hours exposure there remained 

 on evaporation a non-volatile yellow syrup, of a composition 

 not yet determined, which reduced Fehling solution. 



A further study of the artificial photosynthesis of carbo- 

 hydrates was undertaken by Baly, Heilbron and Barker % who 

 by exposing a solution of carbon dioxide in water to ultra- 

 violet light of different wave-lengths obtained evidence of 

 the formation of formaldehyde and also prepared a syrup 

 which, according to Irvine and Francis § consisted of 80 per 

 cent, non-sugars and not more than 10 per cent, of a hexose- 

 like material from which ketoses were absent. 



The views put forward by Baly and his collaborators 

 concerning the origin and mode of formation of the formal- 

 dehyde produced in the experiments mentioned above, were 

 criticized by subsequent workers || and in later papers Baly, 



* Usher and Priestley : *' Proc. Roy. Soc," B, 191 1, 84, 101. 



t Spoehr : " Plant World," 1916, 19, 1. 



J Baly, Heilbron and Barker : " J. Chem. Soc," 1921, 119, 1025. 



§ Irvine and Francis : " Ind. Eng. Chem.," 1924, 16, 1019. 



|| Spoehr : " J. Amer. Chem. Soc," 1923, 45, 1184. Bauer and Reb- 

 mann : " Helv. Chim. Acta.," 1922, 5, 828. Bauer and Biichi : id., 1923, 

 6 » 959- Porter and Ramsperger : " J. Amer. Chem. Soc," 1925, 47, 79. 



