orts led Usher and Priestley, in a series of studies
reported between 1906 and 1911, to submit the whole matter to a critical
review. Briefly, these investigators showed that the photolysis of carbon
dioxide and water results in the formation of formaldehyde and hydrogen
peroxide, as represented by the equation

CO_{2} + 3H_{2}O = CH_{2}O + 2H_{2}O_{2}.

The formaldehyde is then condensed by the protoplasm into sugars, while the
hydrogen peroxide is decomposed, by an enzyme in the plant cell, into water
and oxygen. If the formaldehyde is not used up rapidly enough by the
protoplasm, it kills the enzyme and the undecomposed hydrogen peroxide
destroys the chlorophyll, which stops the whole photosynthetic process.
Usher and Priestley were able to cause the photolysis of carbon dioxide and
water into formaldehyde outside of a green plant, in the presence of a
suitable catalyzing agent which continually destroys the hydrogen peroxide
as fast as it is formed; to show the actual bleaching effect of an excess
of hydrogen peroxide in plant tissues which had been treated in such a way
as to prevent the enzyme from decomposing it; and, finally, to demonstrate
the condensation of formaldehyde into starch by the action of protoplasm
which contained no chlorophyll.

In the meantime, Fenton, in 1907, found that in the presence of magnesium
as a catalyst (it will be shown in Chapter VIII that magnesium is a
constituent of the chlorophyll molecule) formaldehyde may be obtained from
a solution of carbon dioxide in water, especially if weak bases are
present.

Further, Usher and Priestley's later results showed that radium emanations,
acting upon a solution of carbon dioxide in water, produce hydrogen
peroxide and formaldehyde, and the latter polymerizes but not up to the
point represented by the hexose sugars; also, that the ultra-violet rays
from a mercury vapor lamp are very effective in bringing about the
production of hydrogen peroxide and formaldehyde from a saturated aqueous
solution of carbon dioxide, the reaction taking place even in the absence
of any "sensitizer," but much more readily if some "optical" or "chemical"
sensitizer is present. Finally, these investigators were able to duplicate
all their results, using green plant tissues, and to show that the
temperature changes which take place in a film of chlorophyll when it is
exposed to an atmosphere of moist carbon dioxide in the sunlight are such
as woul