periment; he was
ignorant of the fact, notably, that the oxygen is disengaged by plants
only as long as they are under the influence of light.

This important discovery is due to Ingenhouse. Finally, it was
Sennebier who showed that oxygen is obtained from leaves only when
carbonic acid has been introduced into the atmosphere where they
remain. Later on, T. De Saussure and Boussingault inquired into the
conditions most favorable to assimilation. Boussingault demonstrated,
in addition, that the volume of carbonic acid absorbed was equal to
that of the oxygen emitted. Now we know, through a common chemical
experiment, that carbonic acid contains its own volume of oxygen. It
was supposed, then, that carbonic acid was decomposed by sunlight into
carbon and oxygen. Things, however, do not proceed so simply. In fact,
it is certain that, before the complete decomposition into carbon and
oxygen, there comes a moment in which there is oxygen on the one hand
and oxide of carbon (CO_{2} = O + CO) on the other.

The decomposition, having reached this point, can go no further, for
the oxide of carbon is indecomposable by leaves, as the following
experiment proves.

If we put phosphorus and some leaves into an inert gas, such as
hydrogen, we in the first place observe the formation of the white
fumes of phosphoric acid due to the oxidation of the phosphorus by the
oxygen contained in the leaves. This phosphoric acid dissolves in the
water of the test glass and the latter becomes transparent again. If,
now, we introduce some oxide of carbon, we remark in the sun no
formation of phosphoric acid, and this proves that there is no
emission of oxygen.

[Illustration: DEMONSTRATION THAT STARCH IS FORMED IN LEAVES ONLY AT
THE POINTS TOUCHED BY LIGHT.]

This latter hypothesis of the decomposition of carbonic acid into a
half volume of vapor of carbon and one volume of oxygen being
rejected, the idea occurred to consider the carbonic acid in a
hydrated state and to write it CO_{2}HO.

In this case, we should have by the action of chlorophyl: 2CO_{2}HO
(carbonic acid) = 4O (oxygen) + C_{2}H_{2}O_{2} (methylic aldehyde).

This aldehyde is a body that can be polymerized, that is to say, is
capable of combining with itself a certain number of times to form
complexer bodies, especially glucose. This formation of a sugar by
means of methylic aldehyde is not a simple hypothesis, since, on the
one hand, Mr. Loew has executed it by starting fr