82 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
extending over a given interval of time, say a few hours, which it is more 
important to know, and from these it is possible to deduce a mean expression 
for the general equation Ra=(W+w)+7, and for the temperature difference 
6 — @,, or 0, — 8. 
We have recorded in the sequel our endeavours to carry out this idea, but 
the detailed results will be rendered more intelligible by a preliminary 
discussion of a few typical examples which illustrate the principles enunciated 
in the present section of the paper. 
Simultaneous experiments were made on two similar leaves of Helianthus 
annuus for the purpose of determining (1) the rate of photosynthesis of 
earbon dioxide in ordinary air; and (2) the amount of water transpired. 
The leaves were exposed to intermittent sunshine of somewhat low 
intensity for about four hours, the total amount of radiation incident upon 
them being measured by means of the Callendar radiometer placed alongside 
the leaves. 
Tn (1) the leaf was enclosed in a glazed case through which ordinary air 
was passed, and the amount of carbon dioxide assimilated was estimated from 
its determination in the entering and emergent air, the results being 
corrected to the partial pressure of the carbon dioxide in the outer 
atmosphere, which in this instance corresponded to 2°71 parts per 10,000 
of air. 
Leaf (2), for determining the rate of transpiration, was freely exposed 
to the air, which had an average temperature of 16°9 C., and an 
average velocity of 257 kilometres per hour. The partial pressure of the 
water vapour in the air was 9:21 mm. of mercury as determined from the 
average readings of the wet and dry bulb thermometers. The area of each 
leaf was, of course, accurately determined. 
The results were as follows :— 
(R) Average total solar radiation falling on the 
insolated leaves per square centimetre per 
THINUGE Sab Sots seem iet «sale potion eee oer ee 0:2569 calories. 
(A) Coefficient of absorption of the leaves ...... 0°686 
The actual amount of solar energy intercepted was therefore 
Ra = 0:2569 x 0°686 = 0°1762 calorie per square centimetre per minute. 
The amount of carbon dioxide assimilated, reduced to free air conditions, was 
2'134 c.c. per square decimetre per hour, or 0°000355 c.c. per square centimetre 
per minute, which corresponds to an absorption of 0000355 x 5-02 = 0:0017 
calorie per square centimetre per minute (1). 
