754 
Proceedings of the Royal Society 
repartis dans les diverses parties de l’organisme ; que c’est la feuille 
qui les elabore aux depens de l’acid carbon ique et de l’eau.” — 
P. 415, Am. de Chemie, tom xiii. The fundamental chemical 
re-action taking place in the leaf, may therefore be represented 
as follows : — 
(1) C0,0 + H 2 0 - CO,H 2 + 0,0 
(2) 6(CO.H 2 ) = CAA 
In the first equation carbonic acid and water are simultaneously 
attacked with the liberation of a volume of oxygen equal to that 
of the original carbonic, together with the formation of a substance 
having the composition of methylic aldelyde. The second equation 
represents the condensation of this aldelyde into grape sugar. The 
transformation induced in (1) necessitates the absorption of a large 
amount of energy ; and if we neglect the heat evolved in the 
combination of nascent CO and H 2 , which can be shown to be very 
little, the calculated result is made a maximum : whereas the con 
densation of (2) being attended with an evolution of heat, diminishes 
considerably the amount of power required. Happily Frankland’s 
direct determination of the thermal value of grape sugar leaves 
no doubt as to the true equivalent of work done in its formation. 
Taking the following thermal value C0,0 = 68,000, H 2 , O = 68,000, 
C 6 H 12 0 6 # 642,000, 1c centimetre of C0 2 decomposed as in (1) 
would require 6*06 gramme units of heat, or its light equivalent; 
whereas the complete change into grape sugar of the same amount 
of carbonic acid requires only 4 *78 gramme units. But we have 
seen before 1 square decimetre of green leaf functions at the 
rate of 5 *28cc of carbonic acid assimilated per hour, therefore 
(5*28) x (4*78) = 25*23 represents the number of gramme heat 
units conserved through the absorption of light in the above 
period of time. Pouillet estimates the mean total solar radiation 
per square decimetre exposed normally to the sun’s rays in or near 
Paris per hour as 6000 gramme units, so that 6000 - 25*23 = 
represents the fraction of the entire energy conserved. The esti- 
mate is by no means too little, as Boussingault has shown the leaf 
may function at twice the above rate for a limited time. 
In connection with equation (1), above given, as representing 
the action of sunlight on the leaf, it is worthy of remark, that 
