176 Prof. B. Moore and Mr. T. A. Webster. 



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 presence of sodium citrate and sodium carbonate ; this is 

 not reduced at all even on prolonged boiling with formaldehyde, but a 

 solution of "formaldehyde after exposure to ultra-violet light reduces it 

 readily, and the same effect is brought about, though much more slowly, by 

 sunlight. 



This condensation reaction with formaldehyde differs from the synthesis of 

 formaldehyde from carbonic acid and water, in that it does not require the 

 presence of an inorganic activator. In the earlier experiments this was not 

 known, and in these the reduction of the copper salt occurred around the 

 colloidal silicic acid, added as an intended actiVator, showing the interesting 

 fact that the sugar, or reducing substance, had been absorbed by the silica. 

 Later experiments demonstrated, however, that the reaction proceeds with 

 equal or greater rapidity when a solution of 4-5 per cent, of formaldehyde 

 in water only is exposed in quartz tubes to ultra-violet illumination. 



Experiment 1. — A solution containing 5 per cent, of formaldehyde and 

 0'97 per cent, colloidal silicic acid was exposed at 9 cm. distance to the rays 

 from a quartz mercury vapour lamp for a period of 6 hours. At the end of 

 the period a distinct reduction, chiefly in the precipitated silica, was obtained 

 with Benedict's solution. A control, exactly similar, but kept in the dark, 

 gave no reduction. A portion of control solution kept warm in darkness 

 also gave no reduction. 



Experiment 2. — Two quartz tubes were exposed alongside each other for a 

 period of eight hours at 9 cm. distance from lamp ; one contained 5 per cent, 

 formaldehyde in water only, the other 5 per cent, formaldehyde in colloidal 

 silicic acid solution. At the end, the tube with silicic acid reduced 

 Benedict's solution, but to a much less degree than that containing 

 formaldehyde alone, showing that dialysed silicic acid probably acts as an 

 anticatalyst. 



Experiment 3. — A sufficient concentration of the formaldehyde appears to 

 be reached at about 5 per cent., in order to develop the maximum rapidity of 

 formation of the reducing substance. Thus two quartz test-tubes, one 

 containing 40 per cent, and the other 8 per cent, of formaldehyde, were 

 exposed to the quartz mercury-vapour lamp, at the same distance (9 cm.) for 

 the same period (three hours) in each case. Both gave quite a marked 

 reduction, at least as great in the less concentrated solution as in the highly 

 concentrated one. This result, and that of the succeeding experiments 



