90 Dr. H. T. Brown and Mr, F. Escombe. [Jan. 9, 
low, a result which is no doubt due to the low-lying position of Kew Gardens 
and to the absorptive influence of the attenuated veil of London smoke in the 
lower regions of the air. 
When observations are extended over several hours, the mean result, even 
on the clearest day, seldom exceeded about 0°550 calorie per square centi- 
metre per minute, as is shown by Table II. 
The column headed “percentage of sunshine” gives the proportion of the 
total time occupied by the experiment, during which the sunshine was of 
sufficient intensity to produce a record on the Campbell-Stokes’ burning 
recorder of the Kew Observatory. 
(b) The Absorption of Solar Radiation by the Leaf-lamina: Determination of 
a, the “ coefficient of absorption” of the leaf. 
As a necessary preliminary to the discussion of the quantitative relations 
between the energy incident on the leaf and the internal work produced, we 
niust determine the respective proportions of the radiant energy of sunlight 
which are transmitted and absorbed by the leaf-lamina. 
This coeficient of absorption, a value which we have denoted by a in the 
general thermal equations of the leaf referred to in Section (1), was determined 
in the following manner :— 
Over the glass cover of the Callendar’s radiometer was fixed a closely- 
fitting cardboard-cover, out of which a square opening was cut of a sufficient 
size to expose the platinum-spirals, the opening being bisected by the division 
between the spirals. The leaf under experiment, with its petiole dipping into 
a small tube of water, was then laid down on the cover of the radiometer in 
such a manner that the mid-rib* lay between the spirals and the leaf-lamina 
was symmetrically disposed over the coils. Over the leaf there was then 
placed another piece of cardboard with a similar square opening, and when 
this was tied down to the radiometer the lamina was in the right position. 
A favourable day of bright sunshine was selected for the experiment, when 
we could rely on the solar radiation remaining constant, or nearly so, during 
the few minutes required for an observation. 
The plan adopted was in the first place to obtain a measure of the 
unobstructed solar radiation falling normally on the platinum-spirals, waiting 
until the pen of the recorder drew a straight line on the drum. Another 
similar observation was then made alter the interposition of the leaf-lamina, 
* To facilitate the leaf being brought into close apposition to the glass cover of the radiometer, 
slits were cut in opposite sides of the cardboard frame in order to receive the mid-rib. 
