166 Dr. B. Moore and Mr. T. A. Webster. 



below is one devised by Bach,* and later repeated with modifications by 

 Eulerf and by Usher and Priestley.J 



Bach passed a current of carbon dioxide through a solution of 1*5 per cent. 

 crystalloidal uranium acetate, and in the presence of light obtained a 

 precipitate of mixed oxides of uranium, which did not occur when light was 

 excluded during the passage of the carbon dioxide. He adduces no direct 

 experimental evidence of the presence of formaldehyde in the solution, but 

 makes the hypothesis that the carbonic acid by the action of light forms 

 formaldehyde and per-carbonic acid. Then the per-carbonic acid decomposes 

 and forms peroxide of uranium. Lastly, the formaldehyde attacks uranic 

 oxide forming lower oxides recognisable by their colour in the precipitate 

 of mixed oxide. In confirmation he shows that when peroxide of uranium 

 and actually added formaldehyde are exposed to sunlight a reduction to lower 

 oxide occurs with production first of a green and then a violet colour. 



It will be observed, first that the crystalloidal salt of uranium employed 

 undergoes permanent change, and secondly that there is no clear evidence of 

 formation of formaldehyde, although our experiments recorded below show 

 that Bach's hypothesis is probably a correct one. All the experiment 

 actually proves, however, is that acetate of uranium in crystalloidal solution, 

 in presence of carbon dioxide and sunlight, is decomposed, yielding a 

 mixture of oxides of uranium. At the same time it is an important pioneer 

 experiment in this field. 



In a later experiment, Bach obtained evidence of the formation of form- 

 aldehyde by exposing to light a solution of dimethylanibne in dilute 

 sulphuric acid through which carbon dioxide was passed. There is here, 

 however, the objection of using an organic body as catalyst, although the 

 reaction is interesting as a photo-synthesis. 



The same objection holds, as to the presence of the acetic acid anion in 

 the uranium acetate solution used above, since this might serve as a source 

 of formaldehyde. 



A considerable increase in our knowledge was given in the papers of 

 Usher and Priestley quoted above, in which they repeated and extended Bach's 

 experiment. 



The experiments of Bach were repeated and confirmed by these authors, 

 both as to the production of peroxide and formaldehyde. The amount of 

 decomposition obtained in three weeks in bright weather was extremely 

 small, and this was ascribed to the poorness of the uranium as a catalyst, and 



* 'Comptes Eendus,' 1893, vol. 116, p. 1145. 



t 'Berichte d. deut. chem. Gesellsch.,' 1904, Jahrgang 37, vol. 2, p. 3411. 

 J ' Eoy. Soc. Proc.,' 1906, B, vol. 77, p. 369 ; and 1906, B, vol. 78, p. 322. 



