# Page:Popular Science Monthly Volume 80.djvu/527

The first problem was to obtain data regarding the relative influence of the rays of shorter wave-length on different days and in different latitudes; and to obtain reliable figures, it was necessary to determine upon the photocatalytic reaction to be used. The decomposition 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 ${\displaystyle \mu \mu ,}$ to 291 ${\displaystyle \mu \mu }$, was decided upon. Although this method is not free from grave objections, which have been pointed out by me in another article,[1] nevertheless extended experimentation brought the decision 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 surprising. In each one of the localities mentioned above, days of maximum 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 example, 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. Therefore, the difference of climate of these places is not due to geographical location, but is purely a meteorological phenomena. Washington, with a winter climate and presumably much greater atmospheric disturbances, 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.