OXALIC ACID AND URANIUM SALTS AS A PHOTOMETER. 295 



A certain amount of these solutions were now exposed to the sunlight 

 in flat Petri dishes on January 15, 1910, from 8.30 a. m. to 4.30 p. m., 

 w r ith the following result : 



1. Ten cubic centimeters solution No. 1, 68 per cent decomposition. 



2. Ten cubic centimeters solution No. 2, 96 per cent decomposition. 



3. Ten cubic centimeters solution No. 3, 85 per cent decomposition. 



4. Twenty cubic centimeters solution No. 4, completely decomposed. 



Duclaux says that an old solution of oxalic acid is much more sensitive 

 to sunlight than one freshly prepared. He found that he could sensitize 

 his solutions by exposing them to the sun for a comparatively short time 

 and believes this difference to be due to molecular changes taking place 

 in the solution which he compares to sensitized collodion, and to the 

 variations in the degree of rotation of certain sugar solutions for some 

 hours after they are prepared. 



However, my experiments prove that that increase in sensitiveness of 

 the solution is due to nothing else than the formation of hydrogen per- 

 oxide. When this is not possible, the sunlight has no effect on oxalic 

 acicl. 



Duclaux also calls attention to the fact that the decomposition of 

 oxalic acid at first proceeds very slowly, but continues increasing in rate 

 until, during the last two hours of the day, the greatest amount of 

 combustion takes place. The following experiments bring out these 

 facts more clearly : 



January 12, 13, 1910. Manila. Both rather cloudy days. Two Petri dishes 



N 

 each containing 25 cubic centimeters — - oxalic acid were exposed from 8 a. m. to 



4 p. m. during both days. One dish with the same solution was exposed January 

 12 only and one January 13 only. The quantities of oxalic acid decomposed in 

 percentages were as follows : 



January 12 only, 9; January 12 and 13, 69.2; January 13 only, 40; January 12 

 and 13, 70.0. 



Therefore, exposed solutions increase in sensitiveness with the time of 

 exposures, just as Duclaux has stated, the reaction being autocatalytic, 

 as is shown by the fact that the sum of two exposures on one day is not 

 equal to one exposure on two days. 



The following results demonstrate the autocatalytic character of this 

 reaction more clearly : 



January 17, 1910, Manila. Bright day. The dishes similar to the above. 



N 

 Each dish contained 20 cubic centimeters of — oxalic acid. 



Number. 



Time. 



Decom- 

 position. 



1 



a.m. p.m. 

 8.30-2.00 

 8.30^.00 

 8.30-4.00 



Per cent. 

 54 

 97 

 98 



2_ 



3 _ _ 





