

- 
The Thermal Emissivity of a Green Leaf. 137 
The actual difference in the weight of water lost by the two leaf-cases 
during two hours as determined by the balance was 0°290 gramme, and since 
the total leaf surface exposed in each case was 139-4 square centimetres, the 
. 0-290 x 100 
differential loss per square decimetre per hour was 3040S 01040 gramme, 
against 0°1056 gramme, as determined from the differential temperature of 
the leaves. 
It seems probable that this method of determining the rate of transpiration 
by a mere observation of temperature differences may be of service to those 
who are engaged in such investigations, since when once the thermal 
emissivity of the leaf is known, the position of the pen on the drum of the 
recorder enables us to determine at any desired moment the actual amount 
of transpiration which is going on in the leaf. 
VOL. LXXVI.—B. i 
