OXALIC ACID AND URANIUM SALTS AS A PHOTOMETER. 291 



3. Fifteen cubic centimeters 10 per cent oxalic acid solution; 10 of 1 per cent 

 uranyl acetate solution. 1.21 grams oxalic acid decomposed. 



These experiments do not agree very well with each other, but they 

 seem to indicate that there is a limit to the amount of oxalic acid 

 which a definite quantity of uranyl acetate, can decompose, but that 0.1 

 gram uranyl acetate can in some cases decompose as much as fifteen 

 times its weight of oxalic acid. The reaction is therefore catalytic. 



The effect on the absorption of sunlight by the solution of uranium 

 salts on the speed of the reaction was shown by the following: 



A double-walled test tube was prepared, the distance between the two walls 

 being at all points 4 millimeters. Ten cubic centimeters of 10 per cent oxalic 

 acid were placed in the inner tube together with 10 cubic centimeters of 1 per 

 cent uranyl acetate, and the same quantities of these solutions in the outer. The 

 whole apparatus was so designed that the level of the liquid in the outer compart- 

 ment was about 5 millimeters above that in the inner one ; it was exposed to the 

 sun for several hours, and then the decomposition in the two tubes measured. The 

 amount of oxalic acid decomposed in outer tube, 0.234; in the inner, 0.246 gram. 



The experiment was repeated twice with the same general result, 

 namely, the decomposition in the inner tube was always slightly greater 

 than in the outer. This seems very surprising and certainly shows 

 that there is not a very great absorption of the sunlight in a layer 

 of the solution only 1 millimeters thick. It was thought possible that 

 the rays, in passing through a thin layer of a solution of uranium salt 

 might be so changed as to become more active in exciting the reaction; 

 hence a solution of 10 per cent oxalic acid was placed in the inner tube 

 and one of uranyl nitrate in the outer. After a week's exposure to the 

 sun, there was practically no decomposition of the oxalic acid. It is 

 possible that the observed results may be due to a slight lens effect. 

 In any event, the experiment will be repeated in other modifications 

 before an attempt is made to draw conclusions. It is interesting to 

 note, in this connection, that whereas a solution of uranyl acetate will 

 undergo auto-oxidation in the sunlight, and finally deposit a purple 

 to black precipitate, the corresponding nitrate in solution seems to be 

 perfectly stable to sunlight. Similarly, while a limit of the decom- 

 position of oxalic acid by uranyl acetate was apparently reached, some 

 preliminary experiments with the nitrate and sulphate indicate that 

 with these salts the limit is much higher. 



Bach 1T states that he obtained formaldehyde and hydrogen peroxide, by the 

 action of carbon dioxide on water in the presence of uranyl acetate, a result which 

 was strongly criticized by Euler. 18 Nevertheless, it was considered advisable to 



17 Ber. d. deutschen chem. Ges. (1894), 27, 340. 



la lUd. (1904), 37, 3414. Bach's answer, loc. cit, 3985; Ibid. (1906), 39, 

 1672. 



