130 Drs. H. T. Brown and W. E. Wilson. [Jan. 9, 
The application of this method has been found to give very concordant 
results, and although the values have not, perhaps, the degree of precision 
required in the determination of physical constants, they are sufficiently close 
approximations to the true values to be exceedingly useful in any investigation 
of the energetics of the leaf. 
It is scarcely to be expected that the thermal emissivities, even of individual 
leaves of the same plant, should be absolutely identical, and still more might 
we expect variations in leaves of dissimilar plants. Further investiga- 
tion can alone decide the magnitude of these variations, but from the results 
we have obtained with the leaves of the four different species of plants 
mentioned below it is probable that leaves which are not highly glabrous, to 
which we have confined our attention, do not vary greatly in “ emissivity.” 
Thermal Emissivity of Leaves of various Species of Plants under “Still Air” 
Conditions. 

Thermal emissivity in calories per 
square centimetre of surface for 
; a l° C. temperature excess. 
Species of Plant. P 

Per minute. Per second. 
| 
Lnriodendron tulipifera (1) ......... 0:01194 0-000199 
i. Pre Oy Neyer 0:01274 0:000212 
Helianthus multiflorus ............00. 0°01499 0:000249 
Tropeolewin MOUS... 1c. doves ue. 0°01427 0-000237 
LENCO GUIRY OCOD. chen Ssceneoae aces: 0°01598 0:000266 


These emissivities were all determined within a range of temperature of 
from 17° to 19°C. The variation of the coefficient of emissivity with the 
temperature can safely be neglected for any range of temperature to which 
the leaf may be subjected under natural conditions. 
It is worthy of note, in passing, that in their order of magnitude, the above 
values correspond to the emission of heat by a blackened copper sphere 
2 cm. in diameter, cooling in air, as determined by McFarlane.* With a 5° 
temperature difference between the copper ball and its surroundings, he 
which is arranged with the dorsal and the other with the ventral sides outwards, the experiment 
can be repeated after re-arranging the leaves on one coil so that one dorsal and one ventral side is 
exposed. From the difference, if any, in the emission-values so obtained it will be possible to 
ascertain the absolute emissivities of the upper and lower sides of the leaf respectively. 
* © Roy. Soc. Proc.,’ vol. 20, 1871, p. 90. 
