2 FREER. 



The decomposition of oxalic acid into carbon monoxide (re- 

 spectively formic acid), carbon dioxide, and water is brought 

 about almost entirely by those rays in the spectrum of the sun 

 extending from 550 ixfx to 291 /*/x. The sun's spectrum does not 

 extend below the latter point, even in the Tropics. 



That the decomposition is due to these rays is shown by the following 

 summary of data obtained by exposing standard solutions of oxalic acid 

 and uranyl acetate ^ in equal sized (standardized) vessels to the action of 

 the sun during equal periods of time, the vessels being covered with various 

 U-viol glasses, as follows: (1) Without cover; (2) with U-viol 280 a^m; (3) 

 U-viol 289 mm; (4) blue U-viol No. 3653; (5) U-viol copper-ruby No. 2745, 

 all the glasses being 5 millimeters thick. The decomposition in number 1 

 being placed at 1, those taking place in equal times in the others were : 



Number 2 (U-viol 280) 0.888 



Number 3 (U-viol 289) 0.883 



Number 4 (Blue U-viol No. 3653) 0.508 



Number 5 (U-viol copper-ruby No. 2745) 0.004 



The ultra-violet glasses, with the limit slightly beyond the range of the 

 sun's spectrum, exert some absorptive influence; this rapidly increases to 

 the blue, and practical extinction occurs before the red. Similar results 

 were obtained by Bruner and Kozak," who filtered the sunlight through a 

 cold, saturated solution of potassium dichromate of 2 centimeters thickness, 

 and thereby brought the reaction to a standstill. The absorption bands of 

 uranyl salt solutions, according to Deussen,' terminate at 487.5 mm, but the 

 decomposition by light of our standard solution extends below this point, 

 as the cobalt-blue practically terminates at 490 fj-iJ^ and we had minimal 

 decomposition in the red at about 650 ij^ij: However, as we used no glasses 

 between the cobalt-blue and ruby-red, it seems probable that the action of 

 light in the uranyl acetate-oxalic acid solution practically terminates at 

 about 530 iJ-t^, in the green. 



Therefore, so far as the range of its absorption spectrum is 

 concerned, we have in uranyl acetate-oxalic acid a most satis- 

 factory photocatalytic reagent for carrying on the comparative 

 measurements in question. However, although the light rays 

 accelerating the reaction may not extend below, say 530 /x/i, 

 nevertheless, there may be an appreciable heat factor influencing 

 the reaction and due to other causes. 



Bacon,* when he first studied the reaction, concluded from two experi- 

 ments, in which he exposed solutions of uranyl acetate and oxalic acid, at 

 30° and 100° respectively, simultaneously to the sunlight, that no tem- 

 perature coefficient existed over a wide range of temperature, and Bruner 



* Five cubic centimeters of a 1 per cent solution of uranyl acetate, 5 

 cubic centimeters of a 10 per cent oxalic acid solution (crystallized), and 

 20 cubic centimeters of water. 



'Ztschr. f. Elektrochemie (1911), 17, 358. 



'Ann. d. Phys. u. Chem., (Wiedemann) (1898), 66, 1128. 



* This Journal, Sec. A (1910), 5, 290. 



