6 Fowler, Chemical Action of Tropical Sunlight. 



It is evident that the initial reaction does not take 

 place so rapidly in dilute solution, but that on longer 

 exposure the two solutions tend to give results in closer 

 agreement, owing, no doubt, to the greater retarding 

 effect of the iodine liberated in the stronger solution. It 

 will be seen also that very little difference in rate was 

 observable between the 2 hour and i hour period on 

 April 3rd, even with the solution of normal strength. 



As a result of these preliminary experiments, a 

 period of \ hour to i hour was generally chosen for 

 exposure, and the original mixture of loc.c. of sulphuric 

 acid and loc.c. of potassium iodide solution was used as 

 described above. 



An experiment made later, see May 9th (Table III.), 

 confirmed the above results in showing that the rate of 

 liberation of iodine in the half hour or i hour period 

 respectively varied but slightly. 



Systematic Observations. 



The method having been decided upon, a series of 

 comparable observations of the chemical effect of bright 

 sunlight were made in Calcutta and at various places 

 between Calcutta and Marseilles. The object of the 

 observations was in the first place, as already stated, to 

 obtain some measure of the chemical intensity of sunlight 

 in Calcutta, and in the second place to afford additional 

 information to compare with the earlier observations 

 collected by Dr. Bailey. It was further of interest to 

 determine, if possible, whether any relation existed 

 between the chemical effect of tropical sunlight and its 

 liability to cause sunstroke or sunburn. The results of 

 these observations are given in Table III. 



For the purpose of carrying out the observations on 

 the journey from Calcutta to Marseilles, a small box was 



