The Chemistry of Photo-synthesis. 133 
What is the fate of the oxygen separated from the carbon in the 
reduction-process ? (iv.) Is the reduction-process in the cell, wholly 
or partly, a non-vital change? 
In 1904 H. Euler published the first paper of the new era in 
which he proposed to begin his systematic study of the assimilation- 
process by critically repeating and extending the scattered obser¬ 
vations of others. 
He reinvestigated the question of the presence of free form¬ 
aldehyde in assimilation. Polacci had shown that the distillate 
from an extract of assimilating leaves gave a turbidity with anilin 
due to the formation of methylen-anilin. 1 
Euler finds that the extract itself gives very much less reaction 
than the distillate and that an extract of dried hay gives also the 
same reaction. Conclusions as to the presence of free formaldehyde 
in leaves must therefore be drawn only with great caution. Euler 
next repeated the experiments of Bach who had announced that he 
had found CO a reduced to formaldehyde by the action of uranium 
acetate, as an optical sensitiser, in sunlight. This seemed to pro¬ 
vide an inorganic parallel with the action of chlorophyll in the living 
leaf. By doing more careful control experiments than Bach, Euler 
seems to have proved that the same reduction-process takes place— 
though more slowly—in the light, without C0 2 . The action of the 
current of C0 2 passed through the solution of uranium acetate 
seems to have been in removing the oxygen present; nitrogen or 
hydrogen is said to act equally well. 
Bach had also asserted reduction of C0 2 to formaldehyde by 
dimethyl aniline in sunlight, or even in the light of a gas-flame. 
Euler appears to show conclusively that this effect is due to 
impurities in the dimethyl aniline. 
In January of this year Usher and Priestley published a striking 
paper on this question. Their first point is a confirmation of the 
reduction of C0 2 by Uranium Salts in sunlight, asserted by Bach. 
By using highly concentrated C0 2 in a sealed tube they get very 
abundant reduction to formic acid. 
With regard to this result, the reader is very much perplexed 
by the fact that these authors simply do not mention Euler’s work 
or his criticism of Bach’s conclusions. Until these divergent 
opinions have been clearly harmonised a certain suspension of 
1 This is an extremely delicate test for formaldehyde (possibly 
dangerously delicate in view of the assertion that traces of 
formaldehyde always occur in the atmosphere). One part of 
it in 20,000 of water can be detected. 
