324 



Messrs. F. L. Usher and J. H. Priestley. [Apr. 30, 



formic acid, instead of with carbon dioxide. For this purpose a solution of 

 uranyl formate was employed, and the method has the advantage that any 

 removal of formic acid from the system is indicated by a corresponding 

 precipitation of uranic hydroxide. The uranyl formate for these experiments 

 was prepared by heating uranyl nitrate (obtained from Kahlbaum) until 

 oxides of nitrogen were no longer evolved, and dissolving the residual uranic 

 oxide in the least possible quantity of 5-per-cent. formic acid. Preliminary 

 experiments were carried out in order to ascertain whether any visible change 

 takes place when the solution is exposed to sunlight. It was found that on 

 placing a flask containing it in the light, the liquid became turbid after about 

 10 minutes' exposure, and in half an hour a considerable precipitate had 

 accumulated. The precipitate at first produced is of a pure yellow colour, 

 and consists of ordinary uranic hydroxide. After a short time it becomes 

 violet-coloured, and, when a considerable quantity has collected, consists of a 

 mixture of uranic and uranous hydroxides, together with small quantities of 

 uranium peroxide. As in a large number of experiments no formaldehyde 

 could be detected, several experiments were performed with a view to 

 ascertain whether any gaseous products were formed. In one of these, a glass 

 tube about 30 cm. long and 2 cm. bore was half filled with a solution of 

 uranyl formate, drawn out to a capillary, exhausted, and sealed. After four 

 days' exposure to sunlight the tube was connected with a Topler pump and 

 about 50 c.c. of gas were collected, which on analysis was found to consist of 

 approximately equal volumes of carbon dioxide and hydrogen. This 

 experiment was repeated several times, with the same result, though the 

 carbon dioxide and hydrogen were not always produced in the same propor- 

 tions. It should be stated that any uranium salt which remained in solution 

 was invariably reduced to a green uranous compound, and this, no doubt, 

 accounts for the variations observed in the relative volumes of the two gases. 



The foregoing experiments show that carbon dioxide in the presence of 

 uranium compounds gives rise, under the influence of light, to formic acid, 

 and that formic acid under the same conditions gives rise to carbon dioxide 

 and hydrogen. It is probable, for reasons which will appear later, that we 

 are dealing with several consecutive reactions which, under the conditions of 

 the experiments, have widely different velocities, and which are not strictly 

 reversible. If this be so, it follows that it will be very difficult, if not 

 impossible, to isolate certain intermediate products. For example, in the 

 case of the first series of experiments with uranium sulphate, it may be 

 questioned whether the formic acid found really represents the first stage in 

 the photolytic decomposition of carbon dioxide ; it may quite possibly be the 

 final product of a reaction between formaldehyde and one of the hydroxides 



