8 FREER. 



but this average is so high because of the remarkably uniform 

 character of the insolation, the minimum being 14.7 as against 

 9.0 for Kuala Lumpur and 1.15 for Manila. The maximum 

 observed day at Khartoum was 20.8, which is higher than any 

 observation at Manila by 3.0, and 2.7 more than the highest 

 observed at Kuala Lumpur, only two other observed days ap- 

 proaching this, one of 20.7 at Honolulu and the other 20.6 at 

 Baguio in the Philippines, at an altitude of 1,432 meters. In 

 Khartoum, out of sixty-six days of observation, no less than 

 fifty-two gave decompositions between 16.7 and 17.9 and eleven 

 between 17.9 and 18.6. In Khartoum, therefore, we have a 

 remarkably uniform, high insolation so far as the portion of 

 the spectrum under consideration is concerned; but, neverthe- 

 less, the days of maximum illumination do not materially differ 

 from those in the other localities, so that the absolute intensity 

 of the ultra-violet illumination which may reach the earth on 

 perfectly clear days does not materially differ, the distinction 

 being meteorological. If we consider this uniformly high rate 

 and its causes, it is evident that the reverse can also be true 

 and it would be possible to have so-called tropical climates where 

 cloud interference and other causes would bring the average 

 illumination below that in temperate zones. The temperatures 

 of observation at Khartoum were somewhat higher than at 

 Manila and Kuala Lumpur, but we observe that days of maxi- 

 mum temperature are not necessarily days of maximum photo- 

 catalytic decomposition. 



Another interesting comparison is furnished by Bruner and 

 Kozak^' working in Krakau (53° 40' north) on bright, sunshiny 

 days in the spring and summer, the solutions in test tubes being 

 exposed between the hours of 10 and 2. The background is 

 not stated, but as they worked before an open window it is 

 to be presumed that reflections did not play as important a 

 part as with flasks placed on white paper, although the build- 

 ings had to be considered. As the work was done in test tubes, 

 we can not conclusively compare results, the variation owing to 

 the shape of container might be considerable in amount, but 

 still, these authors, with a solution corresponding to our stand- 

 ard, obtained a decomposition of 15 as the maximum in their 

 observations, so that it is apparent that even in this latitude 

 days occur with a photocatalytic reaction suf!iciently high to 

 be comparable with those in the Tropics. 



'' Ihid., 35. 



