86 ACTION OF LIGHT RAYS 



and later by Fischer and Passmore by acting upon concentrated 

 solutions of formaldehyde or other organic substances with caustic 

 alkalies. Thus Loew used formaldehyde and milk of lime, and later 

 freshly precipitated hydroxide of lead, Butlerow dioxymethylene 

 and caustic potash, Fischer and Passmore acrolein bromide and 

 baryta water. 



Such powerful reagents accomplish the condensations and give 

 origin to reducing sugars even in darkness and without need of 

 external supply of energy. In the absence of the alkali which acts 

 as a catalyst, the reaction towards equilibrium is held in check, or 

 proceeds only at an infinitely slow rate, so that a concentrated 

 solution of formaldehyde can be kept in the dark for an indefinite 

 period without developing any reducing substances. 



In nature, no such strong hydroxyl-ion concentration is found in 

 plant tissues as that of the reagents used for synthesis of sugars 

 in vitro as mentioned above; it is therefore interesting to obtain 

 condensation with light exposure in absence of any high alkalinity, 



At the time of these experiments we were unaware of any existing 

 observations on the subject, but we have since discovered a paper 

 by R. Pribram and A. Franke, 1 in which condensation of formalde- 

 hyde and formation of reducing substances was brought about by 

 exposure of concentrated aqueous solutions to ultra-violet light, 

 also a paper by an Italian observer, G. Inghilleri, 2 who exposed a 

 mixture of concentrated aqueous formaldehyde and 6 per cent, 

 of oxalic acid in sealed glass tubes in sunlight for several months, 

 and obtained a hexose which he identified as the sugar sorbose 

 (inactive). In both these cases, although the authors do not com- 

 ment upon it, the law, stated above, of condensation in strong 

 solutions holds. 



Formaldehyde itself reduces Fehling's solution in slight degree 

 after long boiling, though not so readily as any hexose solution. It 

 was therefore of importance to find, if possible, some indicator 

 which would not be reduced by formaldehyde, but would be reduced 

 by more strongly reducing organic substances. This was found in 

 Benedict's solution, an alkaline solution of copper sulphate in pres- 

 ence of sodium citrate and sodium carbonate ; this is not reduced at 



1 Serichte d. D. Chem. Gesellsch., Bd. xliv., p. 1035 (1911), and Monatsh. 

 f. Chem., Bd. xxxiii., p. 415 (1912). 



2 Zeitsch.f. Physiol. Chem., Bd. Ixxi., p. 105 (1911), and Bd. Ixxiii., p. 44 

 (1911). 



