TROPICAL SUNLIGHT 523 



and, therefore, about three years ago, several members of the staff from 

 the two institutions undertook different lines of investigations with the 

 object ultimately of coordinating the results into a monograph on the 

 subject. 



The first problem was to obtain data regarding the relative influ- 

 ence of the rays of shorter wave-length on different days and in dif- 

 ferent latitudes; and to obtain reliable figures, it was necessary to de- 

 termine upon the photocatalytic reaction to be used. The decomposi- 

 tion of oxalic acid in the presence of uranyl salts, which is brought 

 about almost entirely by the rays in the spectrum of the sun extending 

 from 550 /t/i, to 291 fifx, was decided upon. Although this method is not 

 free from grave objections, which have been pointed out by me in 

 another article,^ nevertheless extended experimentation brought the de- 

 cision that it was best adapted to the ends sought. Having decided 

 upon the method and thoroughly learned the factors which influence 

 the reaction, cooperation was requested in a number of laboratories in 

 various parts of the world. The returns have not all been received, but 

 so far extensive comparative measurements have been conducted in 

 Manila (latitude 14° 36'), Kuala Lumpur in the Straits Settlements 

 (latitude 3° 10'), Honolulu (latitude 21° 10'), Washington (38° 59'), 

 Khartoum, Egypt (latitude 14° 36'), and a few data have been obtained 

 from Tucson, Arizona (latitude 32° 12'). As yet, the returns from a 

 number of other places have not been received. The results are sur- 

 prising. In each one of the localities mentioned above, days of maxi- 

 mum insolation were observed which were practically identical, and in 

 a number the averages were very close together. So, for example, at 

 Manila the average percentage of oxalic acid decomposed for one hour 

 during one year was 13.45; at Kuala Lumpur, 11 degrees farther south, 

 15.29 ; at Honolulu, 7 degrees farther north, 13.81, and at Khartoum 

 in the Sudan, 17.6. The great differences do not lie in the averages 

 obtained, but in the minimum observed on cloudy days. So, for ex- 

 ample, both Kuala Lumpur and Khartoum show surprisingly uniform 

 degrees of insolation, whereas in Manila and Honolulu the proportion 

 of cloudy days is so great as to bring the average down materially. In 

 Manila the minimum observed was 1.15; in Honolulu, 3.48; whereas 

 at Kuala Lumpur the same figure was 9, and at Khartoum 14.7. There- 

 fore, the difference of climate of these places is not due to geographical 

 location, but is purely a meteorological phenomena. Washington, with 

 a Avinter climate and presumably much greater atmospheric disturb- 

 ances, on the entire average gave about 33 per cent, less decomposition 

 than Manila, yet the maxima are practically the same and at times 

 Washington shows an astonishingly high average, and there is but little 

 difference, between the summer and winter months in the latter place. 



^PMl. Journ. Sci., Sec. B. (1912), 7, 1. 



