THE STUDY OF TROPICAL SUNLIGHT. 25 



the nights are rarely too hot for comfort and they may even be 

 quite cool. 



Before concluding, I wish to call attention to another phenom- 

 enon to which I referred two years ago in a previous paper on 

 this subject and which, at the time, I said merited further inves- 

 tigation. Bacon ^^ observed that the fall of the aluminium leaves 

 in a fontanoscope, according to Engler and Sieveking, was much 

 accelerated when the apparatus was placed in the sunlight, as 

 compared with the dark. At that time this result was attributed 

 to the ionization of the air by the sunlight of Manila. Since 

 that time, we have modified the apparatus by carefully enclosing 

 it in a glass jar which could thoroughly be dried and which 

 avoided outside influences. Almost 2,000 readings were made 

 both in Manila and Baguio with currents varying from 300 to 

 1,000 liters per hour. The data are too voluminous to quote 

 here, but will be published by Mr. H. D. Gibbs of the laboratory 

 of organic chemistry. Bureau of Science, at a later date. The 

 rate varies from day to day, but with the modified apparatus the 

 highest fall was far below those obtained by Bacon, and a careful 

 analysis shows that the rate during sunlight does not materially 

 differ from that during cloudy weather. Bacon's results can 

 therefore be attributed to outside disturbing factors, as the ap- 

 paratus during Mr. Gibbs's measurements was removed from 

 outside influences. This work is being continued so as to include 

 the total ionization of the air in Manila and will also give the 

 data in regard to the ionization due to radio-activity, the meas- 

 urements for which are being made by Dr. J. R. Wright of the 

 department of physics of the University of the Philippines. 

 Simpson and Wright ^'^ in a study of atmospheric electricity over 

 the ocean, the series extending from the equator southward, did 

 not observe any phenomena which would indicate any unusual 

 ionization of the air by the ultra-violet rays of the sunlight. 



Although the spectrum of the sun as shown by the spectro- 

 graph does not extend beyond 291 ixfi, still it may be possible 

 that we receive rays the nature of which we have not yet de- 

 termined and which, with our present physical technique, we 

 can not determine and which may have an influence in the 

 phenomena of insolation. The discovery of such rays, if they 

 exist, will form an interesting and important chapter in the work 

 on this subject. 



" This Journal, Sec. A (1910), 5, 267. 



" Proc. Roy. Soc. London, Ser. A (1911), 85, 175. 



