88 ACTION OF LIGHT KAYS 



for 2 per cent, solution of formaldehyde is already a concentrated 

 solution possessing as high a molecular concentration as a 12 per 

 cent, solution of a hexose, and the optimum has already been reached. 



Experiment IV. In a similar experiment, four quartz flasks 

 of 250 c.c. each were filled respectively with 0-5, 1, 2, and 4 per cent, 

 solutions of formaldehyde, and exposed to ultra-violet light at 

 7J cms. from the lamp. As a result of the greater volume in the 

 flasks per unit of surface, the appearance of reducing substance is 

 slower than in the quartz test-tubes, but the 2 per cent, solution 

 showed distinct reduction after seven hours' exposure ; the stronger 

 4 per cent, solution and the two more dilute solutions at this time 

 gave no reduction. Tested again after ten hours' total exposure, the 

 J per cent, solution was still negative, while the 1 per cent, showed a 

 faint reduction, the 4 per cent, somewhat more, and the 2 per cent, 

 still gave the best reduction. The J per cent, solution only begins 

 to reduce after about thirty hours' exposure, when all the others 

 are giving a copious reduction. Control flasks kept at 40 C. in 

 a thermostat showed no development of reducing power during 

 this period. 



Experiment V. The formaldehyde used in the above experiments 

 had not recently been distilled, and contained para-formaldehyde. 

 In order to test if the simple aldehyde, or its polymer, or both, gave 

 the reducing substance under the action of the ultra-violet light, 

 a portion of the commercial 40 per cent, solution was distilled, 

 and the first portion coming over, as well as the solid polymer left 

 behind in the retort after distilling over the greater part, were separ- 

 ately made up in aqueous solution, and exposed to light in about 

 4 per cent, solution. The solid was dissolved in distilled water, 

 and the distilled aldehyde was diluted with distilled water. Both 

 were tested with Benedict's solution before exposure, and neither 

 gave any reduction. The two solutions in two quartz test-tubes 

 were then exposed for four hours at 8 cms. distance, and on testing 

 a copious reduction was obtained in both cases. 



Experiment VI. This experiment was mainly designed to locate 

 the wave-length of the light causing the condensation. Two test- 

 tubes were taken, one made of transparent quartz, the other of 

 ordinary soft glass; the dimensions and thicknesses of wall of the 

 tubes were about the same, the capacity of each being about 35 c.c. 

 Each test-tube received 25 c.c. of the same 4 per cent, aqueous 

 solution of formaldehyde which had recently been distilled, and the 



