282 BACON. 



ease there is such a large adaptability, too many conditions which can 

 not exactly be controlled, and, consequently, no one property can be 

 selected which can accurately be measured and known to be influenced 

 alone by the sunlight. Thus, while it is hoped ultimately to extend this 

 investigation to living forms, and while one of the ultimate objects of the 

 researches would be to discover the effects of tropical sunlight on man, 

 it is necessary first to make an extended series of measurements of the 

 effects of this light on the speed of chemical reactions, so that if possible 

 it may be ascertained wherein there is a physical difference in the tropical 

 sunlight as compared with that of temperate zones. The solutions which 

 have been extensively used in this connection as chemical photometers 

 all have some grave defects. 



Bunsen and Roscoe 3 used the 'combination of hydrogen and chlorine. This 

 reaction is exothermic and is also catalyzed by the products of the reaction. 

 While Bunsen and Roscoe largely avoided these disturbing factors by the design 

 used in constructing the apparatus, the method is still open to considerable error, 

 and, furthermore, it is not a convenient one to operate, quite a necessary factor 

 when making extended series of measurements in different places. These authors 

 also used the blackening of a specially prepared silver paper. This reaction 

 suffers from the disadvantage that ready prepared sensitized papers vary greatly, 

 especially in the Tropics where they deteriorate very rapidly, and if special paper 

 is freshly prepared the process is somewhat tedious and it is difficult always to 

 secure a silver salt of the same degree of sensitiveness. Eder 4 proposed the 

 reaction of mercuric chloride on ammonium oxalate, mercurous chloride being 

 separated in the light. As Eder's solution becomes very cloudy in the light, it 

 is obvious that very soon after being exposed to the sunlight, the reaction would 

 proceed quite slowly, as little light can penetrate into the solution. Moreover, 

 the reaction has a marked temperature coefficient ; with a fixed light intensity 

 it proceeds regularly in a slower and slower measure and is very markedly 

 catalyzed 5 by many substances, especially by carbon dioxide, which is one of 

 the products of reaction, and hence is never present in constant amount. There- 

 fore, we must discard Eder's solution as being unfit for our purposes. Marchand ° 

 made a large number of measurements ' with a solution of oxalic and ferric 

 chloride. As I have shown in a prevkms paper,' the action of oxalic acid on 

 ferric chloride in the light proceeds very rapidly during the first few minutes of 

 exposure, but the rate diminishes very soon because of the separation of the iron 

 as insoluble ferrous oxalate. 



Duclaux s used dilute solutions of oxalic acid to measure the effect of sunlight. 



3 Ann. d. Phys. u. Chew. (Poggendorff) (1859), 108, 193; Ibid. (1862), 117, 

 529; Ibid. (1866), 128, 296; Ibid. (1874), 151, 268. 



4 Sitzungsber. Alcad. d. Wiss. math.-not. Klasse, Wien (1879), 80, 636. Eder's 

 solution has 80 grams of ammonium oxalate and 50 grams mercuric chloride in 

 3 liters of water. 



'See also on this point Max Roloff, Ztschr. f. jjhys. Chem. (1894), 13, 327. 

 "Etude sur la force chimique contenue dans la lumiere des soleil. 1875. 

 'This Journal, Sec. A (1907), 2, 129. 



s Atmospheric Actinometry and the Actinic Constitution of the Atmosphere. 

 Smithson. Contrib. Know]. (1903), 29, article 1034: 





