1 18 
Notes . 
The hypothesis I originally formulated to myself (and communicated verbally 
to my friend, Dr. J. Reynolds Green, F.R.S.) was, briefly, that the light rays 
absorbed by chlorophyll are transformed by it into electric energy, and that this 
transformed energy effects the decomposition of carbonic acid (H 2 C 0 3 ) in the 
cell, with the concomitant formation of an aldehyde and the evolution of oxygen. 
To prove the validity of this hypothesis it would appear necessary to establish 
experimentally the truth of the following propositions : — 
1. That an aldehyde is present, even though in very small quantity, in all 
actively photosynthetic tissues, and that the aldehyde is probably formaldehyde. 
2. That the amount of formaldehyde present bears a definite relation to the 
intensity of illumination. 
3. That formaldehyde may be synthetically produced from carbon dioxide in 
presence of water, with evolution of oxygen, by a feeble electric discharge. 
4. That differences in electric potential of sufficient intensity occur in all 
photosynthetic tissues when adequately illuminated. 
5. That the light ray's absorbed by chlorophyll are those chiefly concerned in 
the generation of such electric currents. 
Reserving complete details for a subsequent publication, in which I shall have 
the co-operation of my colleagues, Dr. A. W. Titherley, Lecturer on Organic 
Chemistry, and Dr. F. J. Brislee, Assistant Lecturer in Physical Chemistry, I may at 
present briefly indicate the results obtained as evidence in support of these five 
theses. 
1. Formaldehyde is present , though in very small quantity , in all actively 
photosynthetic tissues : — 
That formaldehyde is a primary product in photosynthesis was first suggested 
by Baeyer in 1870, and its presence in green tissues has been affirmed and denied by 
many investigators since his time. Curtius and Reinke (1897) asserted that an 
aldehyde appeared and disappeared according as the leaf was illuminated or not. 
In 1902 Pollacci affirmed the presence of formaldehyde in the leaf and reiterated this 
statement in 1904, but his latest results have been sceptically received. 
The experimental proof of any theory that postulates formaldehyde as a primary 
product in photosynthesis must obviously depend on the existence of a delicate and 
reliable test for it. A test of this character has recently been published by Mulliken, 
Brown and French (1904) for a reference to which I am indebted to Dr. A. W. 
Titherley, who has latterly been associated with me in the chemistry of this problem. 
A knowledge of this test enabled me to proceed with my work in 1905. If a quite 
fresh and insolated leaf (e. g. Tropaeolwn) be cut into small pieces and shaken up 
with water, the water extract, after filtration, may be shown to contain formaldehyde 
in the following way. To about 1 cc. of a 5 per cent, solution of gallic acid in 
absolute alcohol add about 3 cc. of pure concentrated sulphuric acid so that the two 
layers do not mix, and afterwards allow a small quantity of the filtrate to stream down 
the side of the test tube. If the proper precautions be taken, the presence of 
formaldehyde will be indicated by the appearance of a blue-green ring at the zone of 
contact of the upper and lower liquids. This test has been carefully investigated 
by Dr. Titherley, and we are quite convinced both of its delicacy and of its reliability. 
