1906.] Mechanism of Carbon Assimilation in Green Plants. 323 



and ordinary uranic hydroxides. The insoluble residue* was a hydrate of 

 uranium peroxide, being characterised by its insolubility in acetic acid, and 

 also by the fact that it imparted a brownish-red colour to a solution of 

 titanium oxide in sulphuric acid. The filtrate from the mixed hydroxides 

 was distilled, and the distillate was examined for formaldehyde. None, how- 

 ever, was found, though the liquid energetically 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 at 100°, 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. When treated with phenylhydrazine acetate no 

 crystalline derivative could be separated, nor could the syrup itself be induced 

 to crystallise. On account of its appearance, taste, and reducing action, it 

 presents a striking resemblance to the substance obtained by Butlerowf from 

 formaldehyde and milk of lime, and called by him " methylenitan." It has 

 been shown that formaldehyde, when allowed to stand for some time in 

 presence of uranic hydroxide, gives rise to a substance with similar properties. 

 It does not appear possible to make any more explicit statement with regard 

 to the identity of the substance produced from carbon dioxide in the manner 

 described with methylenitan, since it is produced only in minute quantities, 

 and does not admit of purification, or of the preparation of a derivative which 

 can be purified ; it is, however, worth while to notice that, as regards the two 

 substances produced by the action of light on carbon dioxide in the presence 

 of uranium sulphate on the one hand, and by the condensation of formal- 

 dehyde by uranic oxide on the other : 



(1) They are both organic substances which char on heating. 



(2) Both are insoluble in dry ether, and soluble in water and alcohol. 



(3) Both have a bitter taste, and reduce Fehling's solution. 



(4) Neither reacts with phenylhydrazine. 



Since it was conclusively proved by this experiment and those which 

 preceded it that formic acid is one of the products of the action of sunlight on 

 carbon dioxide, and in view of the fact that the quantity of any product formed 

 is materially influenced by the concentration of the substance giving rise to 

 it — which is necessarily small when that substance is carbon dioxide — it was 

 thouoht that some light would be thrown on the reaction if one started with 



* U0 4 dissolves slowly in acetic acid, if uranous compounds are present, 

 t ' Comptes Eendus,' vol. 53, p. 145 ; ' Liebig's Annalen,' vol. 120, p. 296. 



