ON ORGANIC COMPOUNDS 87 



all even on prolonged boiling with formaldehyde, but a solution of 

 formaldehyde after exposure to ultra-violet light reduces it readily, 

 and the same change 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 pro- 

 ceeds with equal or greater rapidity when a solution of 4 to 5 per cent, 

 of formaldehyde in water only is exposed in quartz tubes to ultra- 

 violet illumination. 



Experiment I. A solution containing 5 per cent, of formaldehyde 

 and 0-97 per cent, colloidal silicic acid was exposed at 9 cms. distance 

 to the rays from a quartz mercury- vapour lamp for a period of six 

 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 II. Two quartz tubes were exposed alongside each 

 other for a period of eight hours at 9 cms. 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 III. 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 cms.) 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 concen- 

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

 (Experiment IV.), does not invalidate the reasoning given above, 



