Action of Light Bays on Organic Compounds. 177 



(Experiment 4), does not invalidate the reasoning given above, 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 4. — In a similar experiment, four quartz flasks of 250 c.c. each 

 vrere filled respectively vyith 0-5, 1, 2, and 4-per-cent. solutions of formalde- 

 hyde, and exposed to ultra-violet light at 7\ cm. from the lamp. As a 

 result of the greater volume in the flasks per unit of surface, the appearance 

 of reducing substance is slov?er 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 10 hours' total exposure, the ^-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 |-per-cent. solution only begins to reduce after about 

 30 hours' exposure, when all the others are giving a copious reduction. 

 Control flasks kept at 40° C. in thermostat showed no development of 

 reducing power during this period. 



Experiment 5. — 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 separately made up in aqueous solution, and exposed to 

 light in about 4-per-cem}. 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 cm. distance, and, on testing, a copious reduction was 

 obtained in both cases. 



Experiment 6. — 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 two tubes were placed in equally favourable positions 

 alongside each other at about 5 cm. distance from the quartz tube of a 

 mercury-vapour lamp. At the outset the formaldehyde gave no reduction of 



