4 Fowler, Chemical Action of Tropical Sunlight. 



Effect of Duration of Exposure. 



The next point to consider was the effect of variations 

 in the duration of the exposure, and in this connection it 

 was important to determine the effect of the liberated 

 iodine in absorbing the chemically active rays. 



For this purpose two bottles were exposed, one in 

 the usual manner, and the other completely immersed in 

 a centinormal solution of iodine contained in a small 

 beaker. The following results were obtained : — 



Table I. 

 Effect of Duration of Exposure. 



The above results show clearly the absorbing effect of 

 the iodine solution, and, in consequence, no doubt, of this, 

 the rate of change during the three hours exposure on 

 March 29th is appreciably less than during the one hour 

 exposure on March 30th, both being in full sun. This is 

 accentuated in an experiment made later, when the usual 

 solution was exposed from 8-45 a.m. on April 18th to 

 6 p.m. on April 19th, a total of 33^ hours comprising 

 about 20 hours of full sun. In this case i6'5 c.c. thio were 

 required corresponding to an average rate for the entire 

 period of only 4*9 mgms. iodine per 100 c.c. per hour, or, 

 assuming2ohours sunshine, of 8*25 mgms.iodine per 100 c.c. 

 perhour of sunshine, as against 26 mgms. on March 30th. 



