OXALIC ACID AND URANIUM SALTS AS A PHOTOMETER. 293 



May 12, which he classifies as a "fine day," gave a combustion of 46 per cent, 

 while May 13, given as a "very fine clay," gave a combustion of only 29 per cent. 

 September 6 and 7 each gave one of 7 per cent, while September 8, classed as a 

 "day similar in appearance to the two preceding," gave a combustion of 28 

 per cent. 



.Man}- similar figures could be given. Every observer of the effect of 

 sunlight on chemical reactions from the time of Bunsen and Koscoe, 

 has noted that without regard to the general appearance of the lumi- 

 nosity of the sun to the eye, or to the appearance of the sky and the 

 transparency of the air, there are days which must be classed as actinic 

 and others as nonactinic. Duclaux ' considers that so-called "invisible 

 actinic clouds of vegetable matter*' are formed on certain days which 

 absorb the actinic rays of the sun and are oxidized, preventing their 

 reaching the surface of the earth. This author measured the decomposi- 

 tion of oxalic acid in France, in Helsingfors and in Algeria, and found 

 that in traveling north, the rate of the reaction becomes greater. He 

 says "It (the actinic quality of the sun) would no doubt be found still 

 weaker if we approached more nearly to the equator."' 



My measurements, made according to Duclaux's methods do not bear 

 out this statement, as the rate of decomposition of oxalic acid in Manila 

 is much greater than that which he found for any locality in which 

 measurements were made. I consider one of the main factors in causing 

 actinic and nonactinic days to be quite different from that assigned by 

 Duclaux, for I regard a day with a homogeneous atmosphere as actinic, 

 and one during which the atmosphere is more or less stratified as non- 

 actinic. It is well known that a comparatively short thickness of air 

 will cut out a large amount of the more refrangible rays of light, if 

 this air is composed of a series of layers of different densities. The 

 measurements of Duclaux in Algeria were made on the edge of a desert, 

 where the conditions are ideal for forming columns of heated air and 

 ultimately many strata of very different densities. The desert is the 

 place par excellence' of optical illusions, these having their physical 

 basis in such air layers. One would not expect the refrangible, short 

 waves of light to reach the earth in quantity in such a region. The 

 measurements at Helsingfors were on the seashore, where the great body 

 of water tends to make the air homogeneous, while those in the interior 

 of France, with a considerable amount of vegetation, would be expected 

 to be intermediate between the ones at Helsingfors and Algeria. Trop- 

 ical islands almost absolutely covered with vegetation and surrounded 

 by the sea offer the least opportunity for the production of columns of 

 heated air and therefore stratification, as there are practically no bare 

 surfaces of the earth to give rise to heated layers. Moreover, this 



