372 SCIENCE PROGRESS 



absorbs light of shorter wave length than ZS^I^t^t the vital 

 differing, therefore, from the laboratory process, by making use 

 of light of longer wave length. The authors have found that 

 this difference can be overcome by the use of a " photo-catalyst " 

 and have been able to synthesise sugars from carbon dioxide 

 and water in two stages under the influence of light such as is 

 used by the living plant. In the case of the synthesis of formal- 

 dehyde, a suitable photo-catalyst has been found in a coloured 

 basic compound, such as malachite green or p-nitroso dime- 

 thylaniline ; an aqueous solution of either of these compounds 

 saturated with carbon dioxide gives formaldehyde on exposure 

 to light behind a thick plate-glass screen, but no formaldehyde 

 is produced under similar conditions in the absence of carbon 

 dioxide. These results are in contradiction to those of Oster- 

 hout {Amer. J. Bot., 191 8, 5, 511), who found that moist filter 

 paper, dyed with aniline green or methyl green, when exposed 

 to sunlight under glass, gave rise to formaldehyde, but that the 

 presence or absence of carbon dioxide was without influence, 

 but that oxygen was essential ; but the contradiction is prob- 

 ably more apparent than real, as the experimental conditions 

 were very different. The photo-catalysis of the synthesis of 

 reducing sugars is brought about by a complex copper ion. An 

 aqueous solution containing 17 per cent, of sodium citrate 

 and 9 per cent, of sodium carbonate, on exposure to ultra-violet 

 light, gives formaldehyde, and enough sugar to cause marked 

 reduction of a i per cent, solution of copper sulphate when 

 added, but no sugar is formed if the solution is exposed under 

 a screen of thick plate-glass. If, however, the copper sulphate 

 is added to the solution first, marked reduction is obtained 

 after exposure under plate glass. 



Moore and Webster's discovery, that many substances of 

 bio-chemical origin give formaldehyde when exposed in aqueous 

 solution to ultra-violet light, is apt to convey the impression 

 that carbohydrates cannot be built up from this compound 

 by the plant ; but such compounds as acetone under these con- 

 ditions give rise, not only to formaldehyde, but also to reducing 

 sugars formed in each case through carbon dioxide, the primary 

 product of the action of light. 



It seems that the effect of ultra-violet light is to set up an 

 equilibrium between sugar, carbon dioxide, and formaldehyde 

 lying far over to the side of carbon dioxide ; the introduction, 

 however, of a photo-catalyst which carries the reaction from 

 carbon dioxide and water to sugar in visible light will shift the 

 equilibrium right over, as the system will be screened from those 

 rays which decompose the carbohydrate. 



The authors have not yet found a photo-catalyst capable 

 of acting in both reactions, the synthesis and polymerisation of 



