

1905. | On the Thermal Emissivity of a Green Leaf. 125 
and of a length of about 2°4 metres, so arranged as to form a flat grid for the 
leaf-lamina to rest upon. The ends of the platinum coil were attached to two 
small brass lugs screwed on to the outer ebonite frame, and these could 
be connected with the leads of the recording instrument by suitable means. 
Two other small movable frames of ebonite were constructed, which fitted 
accurately into either side of the outer frame, and rested, when in position, on 
the projecting edges of the mica-plate. Across these inner frames were laced 
thin silk threads for the purpose of lightly pressing the leaf-lamina against the 
platinum coil. 
In using this instrument two similar leaves were always used, these being 
cut to the proper dimensions for placing in the outer frame. When the inner 
frames were brought into position, and fixed by means of small brass buttons 
attached to the outer frame, the platinum-resistance-coil was enclosed by the 
two leaf-lamine in close apposition, and was very favourably placed for 
rapidly acquiring the mean temperature of the leaves. 
Since the leaves had to be freely supplied with water, provision had to be 
made for their petioles to dip into small tubes of water, and for this purpose a 
portion of the lower part of each of the ebonite frames was cut away and 
bridged over with a thin curved piece of sheet brass of suitable dimensions. 
The leaf-stalks passed through a split cork which closed the mouth of the 
small water-tubes, and this was made tight with a mixture of paraffin and 
vaseline.* It will be noticed that the leaf surface exposed in this apparatus 
is always the same. The area of the free space of the inner frame was, in this 
particular case, 69°72 sq. cm., so that the total leaf surface exposed was 
139-44 sq. cm. 
The upper part of the outer frame was furnished with a loop of wire, 
by which the apparatus could be suspended to the arm of the balance when 
it had to be weighed. 
Two exactly similar sets of frames were constructed, each of which, when 
mounted with their leaves and water-tubes, weighed about 60 grammes. 
The difference in temperature between the two coils was determined by 
means of the Callendar recorder, and the resistance of the coils, which 
amounted to about 13:8 ohms at 18°5 C., was very accurately adjusted for a 
“fundamental interval” of 5 ohms between 0° ©. and 100° C., so that when a 
No. 4 bridge wire was used in the recorder, each scale division of 4 mm. 
* After the cork around the petiole has thus been rendered impervious by the wax, care has to 
taken to pierce this with a fine needle in order to allow the equalisation of pressure between the 
outer air and the air of the tube. If this precaution is not taken there is some danger of the free 
ascent of the water being impeded. 
