

On the Thermal Emissinty of a Green Leaf: 123 
inapplicable to this particular case, we devised an indirect method which has 
given very good results, and is capable, we believe, of still further extension 
and refinement. It is based on the following principles :— 
A leaf which is actively transpiring tends, by a process of self-cooling, 
to become lower in temperature than its surroundings. Assuming such a leaf 
to be placed in an enclosure, the walls and air of which are kept at constant 
temperature, whilst the air is maintained at a uniform state of humidity, the 
temperature of the leaf falls to a definite point below that of its surroundings, 
and then becomes stationary, providing the openings of the stomates do not 
alter. When this steady thermal condition is reached, the internal work of 
vaporisation manifestly becomes a measure of the amount of energy flowing 
into the leaf from its surroundings, providing we neglect, as we may safely do, 
the very small amount of heat self-produced in the leaf by the respiratory 
process.* 
If,now, we have the means of determining, (1) the amount of water lost by 
the leaf in a given time by vaporisation ; (2) the area of the leaf surface, and 
(3) the exact difference of temperature between the leaf-lamina and its 
surroundings, we have all the data necessary for ascertaining the amount 
of energy in water-gramme-units (calories) which flows into the leaf per unit- 
area, of surface and unit-time for a 1° excess of temperature in the surround- 
ings. But this is the same thing as the thermal emissivity, since emission and 
absorption are equal under the same relative conditions. 
The range of temperature of any experiments of this nature is of course 
small, and it was therefore necessary to devise a form of apparatus, 
by means of which the temperature difference between the leaf and its 
environment could be accurately determined. 
The thermometric apparatus used. consisted of two differential platinum- 
resistance-thermometers, constructed for us by the Cambridge Instrument Co., 
and we must here express our great indebtedness to Professor Callendar for 
his help in designing the apparatus, and for most valuable assistance in other 
directions during the course of the investigation. The arrangement may be 
briefly described as follows :— 
Two exactly similar frames of very thin ebonite, which in this particular 
instance had outside dimensions of 11:°9x11°3 cm. and a width of 0°7 cm., 
were screwed tightly together on either side of a thin frame of mica which 
projected a few millimetres into the open part of the ebonite frame. This 
inner fringing edge of mica-plate was pierced top and bottom with a row of 
fine holes, through which was threaded a platinum wire of 0-006-inch gauge, 
* Cf. Brown and Escombe, ‘ Roy. Soc. Proc.,’ this vol., p. 71. 
