4 FREER. 



and character of the flasks used. Uranyl acetate-oxalic acid 

 solution exposed on a surface of glazed black paper was de- 

 composed in a ratio of 0.73 to 1 for glazed white paper and 

 0.74 for a black, dull background. Although there is practically 

 no difference between a glazed black or a dull black background, 

 it is better to adhere to one kind; therefore, the dull paper was 

 selected. 



A much greater variation is brought about by the materials 

 of the flasks used, as well as by their size and shape. Obviously, 

 a quartz flask is best, as it allows the ultra-violet rays to pass 

 with a minimum of absorption, but the difference in absorption 

 between quartz and Jena glass is not so great as might be sup- 

 posed. Of greater influence is the size and shape of the vessel, 

 that is, of the surface of liquid exposed. Bacon * has already 

 pointed out that the rate of decomposition increases with the 

 size of the flask ; but even between flasks of the same capacity a 

 variation is found which can readily be understood when we 

 realize that two 100-cubic-centimeter Erlenmeyer flasks of Jena 

 glass may differ 50 per cent in weight. Two such vessels, of 

 equal capacity, of 100 cubic centimeters or under do not vary 

 more than 2 per cent, and standardized flasks were used in our 

 measurements, wherever possible. 



Bacon ^ showed that the concentration of oxalic acid, except- 

 ing at great dilution, does not influence that speed of the reaction, 

 and Bruner and Kozak have confirmed this result. Obviously, 

 when decomposition of the acid reaches a point where its con- 

 centration has diminished below the critical one, the reaction 

 will gradually diminish in rate; however, this point, in the 

 solutions used for comparative measurements is not reached 

 until more than 60 per cent of the acid has been decomposed, 

 a number rarely reached in three-hour exposures, and even then 

 the initial diminution is small, but the fact must be taken into 

 consideration where longer exposures are resorted to. 



To sum up. The decomposition of a solution of uranyl acetate- 

 oxalic acid by the sunlight is by no means a perfect indicator, 

 for comparative purposes, of the total ultra-violet photocatalytic 

 effect of the sunlight, for it suffers from the errors outlined 

 above; but when the nature of the measurements is taken into 

 consideration and when we consider that the object to be attained 

 is a knowledge of the average influence of the sunlight during 

 long periods in various latitudes, small individual errors can be 



' Loc. cit., 288. " Loc. cit., 285. 



