136 The Thermal Enusswity of a Green Leaf. 
h, the latent heat of vaporisation of water at the temperature of the air™ 
expressed in water-gramme-units ; 
(@—86,,) the temperature difference observed ; and 
e the thermal emissivity of the leaf in calories per square centimetre of 
leaf surface, per minute, for a 1° C. temperature excess. 
e(@—8,) 
Then (1) ¢ Qh (2) Q=S5 
ri 2 On 
~ (O—6,)’ , (3) 0-4) = =. 
Having once established and plotted the values of ¢ for a particular kind 
of leaf, and for given conditions of air-movement it becomes possible by means 
of equation (2) to translate the temperature-records of the drum of the 
recorder, for any period, or for any particular moment of time, into grammes 
of water transpired per unit-area, and unit-time. If we are measuring the 
difference of temperature between the leaf and the surrounding air, it is the 
total loss of water by the leaf which is thus measured, whilst on the other 
hand, if both thermometer-coils are furnished with leaves, it is of course only 
the differential transpiration-effect which is determined. 
As an example, we may take an experiment on the differential transpiration 
of two pairs of leaves of Liriodendron tulipifera in moving air, the details of 
which were as follows :— 
Time occupied by the experiment, two hours. 
Temperature of the air, 1875 C. 
Latent heat of vaporisation (1) = 593°7 calories. 
Mean differential temperature of the pairs of leaves (@—@,) =0°45 C. 
Velocity of air-current 71°4 metres per minute. 
The thermal emissivity (¢) of this leaf in an air-current of the above 
velocity 1s 
e = 0:0119+(0:000174 x 71:4) = 0:02483. 
From equation (2) the mean differential transpiration per square centimetre 
of leaf-lamina per minute, will be 
00243 x 0°45 
(Qj hee ee 
593-7 = 0:0000176 gramme. 
This is equivalent to a differential transpiration of 0°1056 gramme per 
square decimetre per hour. 
* The latent heat of vaporisation is represented by 606°5 —0°695 0, where @ is the temperature 
of the air. Strictly speaking it is the temperature of the transpiring leaf which should be taken 
to represent @, or, in the case of differential transpiration, the mean temperature of the two sets of 
leaves, but the error introduced from this cause is insignificant, amounting to not more than 
about O'l per cent. 
