42 SYNTHESIS BY SUNLIGHT IN RELATIONSHIP 



A precipitate appeared after 15 minutes' exposure, and in 24 hours 

 the reaction was complete. The tubes on opening were found to 

 contain uranium peroxide and formic acid, but no formaldehyde. 



Usher and Priestley then repeated their experiments, using uran- 

 ium sulphate instead of the acetate. Carbon dioxide was passed 

 through a 2 per cent, crystalloidal uranium sulphate solution, and 

 the solution was exposed to sunlight on a roof for nearly a fortnight. 

 Several grammes of a precipitate of mixed oxides of uranium were 

 obtained of a pale violet colour. The greater part was a mixture 

 of uranous and ordinary uranic hydroxides soluble in acetic acid. 

 The insoluble residue was a hydrate of uranium peroxide. The 

 nitrate from all these mixed hydroxides was distilled and examined 

 for formaldehyde. None, however, was found, though the liquid 

 reduced Fehling's solution and silver nitrate. It was subsequently 

 found to contain formic acid, the lead salt of which was prepared 

 and identified. The undistilled residue was then evaporated down, 

 and when nearly solid was repeatedly extracted with dry ether in 

 order to remove any formic acid which had not evaporated. The 

 residual solid was extracted with absolute alcohol, and the solution 

 on evaporation left a small quantity of a brown syrup, bitter to the 

 taste, which reduced Fehling's solution. It could not be proven 

 that this formed an osazone, but it closely resembled in its properties 

 a substance called " methylenitan " obtained by Butlerow from 

 formaldehyde and milk of lime. The body was obtained in minute 

 amount only. 



This experiment constitutes a distinct advance, since the organic 

 substances (formic acid and the body above mentioned) were ob- 

 tained by the action of light on purely inorganic substances. 



The points still left against the results, from the aspect mentioned 

 at the outset of the present chapter, are that the catalyst is an ex- 

 tremely rare one in nature, that it was used in high concentration in 

 crystalloidal solution, and that it underwent changes in itself and 

 was precipitated as the result of the reaction. The fact that formic 

 acid was obtained instead of formaldehyde, looked at from our 

 point of view, is relatively unimportant, since both are organic 

 bodies of increased energy content. 



By the use of many times more dilute colloidal uranic hydrox- 

 ide, we have been able now to obtain formaldehyde, and this 

 without precipitation or other visible change in our catalyst. Com- 

 pared with a stronger solution of crystalloidal uranium nitrate 



