THE TROPICAL SUNLIGHT. 



However, other chemical reactions could be selected, perhaps with less 

 experimental difficulty, provided all side reactions, such as oxidation or 

 peroxide formation, were eliminated. Such a reaction is the decomposi- 

 tion of oxalic acid in the presence of uranium acetate as a catalyzer, the 

 gas evolved in a given time by given rays being measured, or the residual 

 oxalic acid titrated. This reagent for the measurement of the two factors 

 of the sunlight without, however, as yet using definite lines, has been 

 utilized by Dr. Raymond F. Bacon, of the Bureau of Science, he being able 

 to compare the insolation at Chicago, in June, with that of Manila 

 throughout the year. I will discuss his results in another portion of this 

 paper. The method being fully developed, subsequent experiments, using 

 definite lines of the spectrum will be a simple matter. 



His measurements, although extending over a limited period in Chi- 

 cago, are nevertheless of such a nature that we have an experimental 

 comparison between the effect of' insolation in the Tropics and at a 

 northern point. Previous attempts have been made to calculate the 

 effect from the data of Bunsen and Boscoe, 7 taking into consideration the 

 direct and diffused sunlight. The calculations result in the development 

 of numbers showing that "diffused light tends to equalize the numbers 

 for the total quantity of light at different latitudes." So, according to 

 Sebeliene, 8 while "the daily quantity of light due to direct radiation is 

 forty times as great at the equator as it is at the pole, the quantity of 

 diffused daylight is hardly twice as great at the equator as at the pole on 

 the same day." The results of Sebeliene's calculations, based on Bunsen 

 and Roscoe's figures, give a total quantity of light units' at 0° of latitude 

 as 82,716, a maximum of 114,835 at 30°, and a minimum of 76,048 at 

 the pole. However, these calculations refer only to the midsummer day, 

 and certainly lack the basis of experiment in various parts of the world. 

 While the northern and southern parts of the globe are much more for- 

 tunately situated at midsummer in respect to light, as compared with 

 the Tropics, this would not hold good throughout the year. Bacon, in 

 the Bureau of Science, found the decomposition of oxalic acid in the 

 presence of uranium acetate to proceed approximately five times as fast 

 in Manila in October and November as in Chicago during the months of 

 May and June ; and recent days in February have shown a rate as high as 

 twenty. Bunsen and Boscoe ° also demonstrated that days of light, hazy 

 cloud, through which the sun just shines, are able remarkably to increase 

 the chemical activity of light. 



Enough has been said to demonstrate the difficulties to be encountered 

 in securing data fit for comparison; the experimental work so far ac- 

 complished, therefore, is as yet only tentative and must be taken for 

 what it is worth. 



7 Loc. cit. (1S59), 108, 257, 2G0. 

 sphil. Mag. (1905) (VI), 9, 354. 

 "Log. cit. (1859), 108, 236. 



