OXALIC ACID AND URANIUM SALTS AS A PHOTOMETER. 283 



I shall show below that the decomposition of oxalic acid, as he used it, is due 

 to an intermediate formation of hydrogen peroxide, and is so markedly autocata- 

 lytie that over half the decomposition would often take place during the last 

 two hours of the afternoon, when the sun's power is quite feeble. Hence, this 

 solution must also be discarded, although I made a few measurements with it 

 in Manila to compare them with Duclaux's results in France and elsewhere. 



In a previous paper," in which I made some brief studies on the action 

 of uranium salts on oxalic acid to determine whether radioactivity was 

 connected with the observed effect, I stated that this solution appeared 

 to be very promising as a chemical photometer. Further work has con- 

 firmed this conclusion, and while it is not ideal from every standpoint, 

 still it seems to lie the best for the purpose in hand of any which have 

 been suggested. 



Oxalic acid 10 solutions absorb almost all of the rays of the ultra-violet 

 spectrum, and the same is true of uranium salts, so that the ultraviolet 

 portion of the- sun's rays may usually be assumed to be that part which 

 is active in the reaction. TCayser, 11 in summing up the work on the 

 absorption spectrum of uranyl salt solutions, states that the relations 

 are evidently very complicated, but that there is general absorption in 

 the violet and ultra-violet regions with bands in the green, blue, and 

 violet. Jones and Strong 12 state in a very recent article that uranyl 

 salts in solution are yellow and their absorption spectra consist of a 

 broad band of general absorption in the ultra-violet, which extends more 

 and more into the region of longer wave lengths as the amount of 

 uranyl salt solution in the beam of light is increased. 



Therefore, it may be considered as being fairly well established that 

 the active waves from the sun measured by this solution are in the ultra- 

 violet. T have shown in another paper ,3 that there are good grounds 



9 hoc. cit. 



10 According to Hartley and Huntington, Phil. Trans. (1879), 170, I, 257-274, 



with a 10 per cent solution of oxalic acid absorption begins at wave length 



320 ix ix. According to R. Mazini, Nnovo Chim. (1903), (5), 6, 343-370, who 



used a layer 1 centimeter in thickness, absorption begins for the different dilutions 



as follows: 



N N N N 



p 300.9, ^, 270.6, ±, 234.4, ^ 230.0. 



11 Handbuch, III, 426. A good summary of the literature is found in Kayser's 

 Handbuch III, 418. 



r -Anter. Chem. Journ. (1910), 43, 46. No work seems to have been done on 

 the infra-red absorption spectra of oxalic acid or uranium salt solutions. See 

 Coblentz, Investigations of Infra-Red Spectra. Carnegie Inst. Washington, 1905, 

 1906, 1908. The fact shown later that the uranyl acetate-oxalic acid reaction has 

 no temperature coefficient seems to indicate that there are no active rays involved 

 in this part of the spectrum. 



"The Ionization of the Air by Tropical Sunlight with Some Notes on Radio- 

 active Phenomena in the Philippines. This Journal, Sec. A (1910), 5, 



