EFFECT OF TROPICAL SUNLIGHT ON THE ATMOSPHERE. 269 



III. — In sunlight. 



Date. 



February 5 (») 

 February 5 ( b ) 



Time. 



Poten- 

 tial, elec- 

 troscope. 



February 14 (°). 



February 14 {<*). 



February 14 . 



o. m. 



9. -21 



10.01 



10.10 



11.10 



9.15 



9.30 



9.45 



10.10 



10.25 



10.40 



10.57 



11.27 



11.55 



12. 55 



1.20 



1.50 



115.7 

 90.5 

 128.1 

 82.3 

 177.1 

 167.1 

 161.1 

 159. 9 

 154.9 

 146.9 

 158.2 

 142.5 

 134.1 

 98.2 

 169.9 

 148.4 



Fall, 



volts per 



hour. 



37.8 

 45.8 



32.0 



20.0 

 32.0 



31.4 

 35.9 

 43.0 



Air 



current 

 per hour. 



Liters. 

 1,000 



1,000 

 540 



1,000 

 1,01)0 



1,000 

 1,000 

 1,000 



• The sun was behind a cloud for five minutes during these measurements. 



b During the first ten minutes of this measurement the sun was wholly unclouded and 

 very bright, and during that time the electroscope fell at the rate of 76.2 volts per hour. 

 During the remainder of the time the sun was intermittently covered with clouds. 



c The sun was behind clouds about one-quarter of the time. During the last fifteen 

 minutes it was behind clouds aoout one-half of the time. 



d During the whole of the first fifteen minutes of this measurement the sun was behind a 

 cloud of such density that the disc of the sun was just visible. During the second fifteen 

 minutes the sun was behind dense clouds about half the time, and the other half it shone 

 from a clear sky. The effect caused by the sun going behind a cloud was easily discernible 

 in the instrument. 



Average for 1 sunlight, 35.7 volts. 



The results seem to indicate with considerable clearness that the 

 tropical sunlight ionizes the air. I am well aware that much more 

 work must be done before a positive statement can be made on this sub- 

 ject, especially as such phenomena do not seem to take place in temperate 

 zones, although it is well known from the work of Senard 3 and others 

 that certain forms of ultra-violet light do ionize gases to a marked degree, 

 even though Buisson, 4 working at an earlier date, was unable to detect 

 any conductivity in air through which ultra-violet light was passing. 

 Buisson's original paper is not available to me, so that I do not know 

 the exact conditions under which he worked, and therefore am not able 

 to discuss his results which seem to contradict those obtained by me. 

 However, I incline to the view that the ionization observed may not be 

 a direct effect of the tropical sunlight on the air, but may be a secondary 

 phenomenon connected with the large amount of moisture in the air of 

 tropical islands and with the luxuriant vegetation of this region. Thus, 



3 Drudes Ann. (1898), 1, 483; Ibid. (1900), 3, 298. 



4 Quoted by Perm, in Ann. chim. et phys., Paris (1897) 



(7), 11, 526. 



