On the thermal emissivity of a green leaf in still and moving air

Abstract

During the recent investigation of the energies of a foliage leaf, 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 temperature, 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 emissivity 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 temperature 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 of a copper ball, or those of 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, 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.