122 
On the Thermal Emissivity of a Green Leaf in Still and 
Moving Arr. 
By Horace T. Brown, LL.D., F.R.S., and W. E. Witson, D.Sc., FBS. 
Received January 9,—Read March 23, 1905. 
(From the Physical Laboratory of the Daramona Observatory, Westmeath, Ireland.) 
During the recent investigation of the energetics of a foliage leat,* it was 
necessary to acquire some knowledge of the rate of interchange of heat 
between the leaf-lamina and its surroundings for a known excess of tempera- 
ture, and under given conditions as regards the movement of the surrounding 
air. It was required, in fact, to determine with some approach to accuracy, 
the thermal enissivity in air in absolute units, including in this term the loss 
of heat due both to radiation and the conductive and convective properties of 
the surrounding air. The thermal emissivity is an important factor in the 
economy of the living plant, since it determines both the maximum tempera- 
ture to which the leaf can be raised above its surroundings in those cases 
where the incident radiation is more than sufficient to perform the internal 
work of the leaf, and also the extent to which the leaf can be cooled below its 
surroundings when the receipt of radiant energy falls short of that required to 
produce the observed internal work. 
There are comparatively few determinations in absolute units of the thermal 
emissivity of bodies cooling in air, and the results of experiments such as those 
of McFarlane on the cooling cf a copper ball, or those cf J. T. Bottomley, 
Schleiermacher, and Ayrton and Kilgour on the emissivity of platinum wires, 
cannot be rendered applicable to our requirements, owing in the first place to 
the nature of the emitting substances differing so widely from that of a leaf- 
lamina, and secondly to the fact, emphasized by the experiments of Ayrton 
and Kilgour,f that the loss of heat from radiation and air convection per 
square centimetre of surface per one degree excess of temperature is by no 
means constant, even for substances of a similar nature, and varies greatly 
with the size and shape of the cooling body. 
In the absence of data from which we could deduce even the order of 
magnitude of the thermal emissivity of a leaf-lainina, it became necessary to 
attack the problem experimentally, and since the ordinary methods employed 
for determining the “rate of cooling” of a heated body are manifestly 
* See Brown and Escombeg, ‘ Roy. Soc. Proc.,’ this vol., p. 29. 
T Phil. Trans.,’ A, vol. 183, 1893, p. 371. 
