188 Scientifie Proceedings, Royal Dublin Society. 
in full foliage should, we imagine, expose more than 1 square 
metre of leaf surface to every square metre of land surface it 
covers. Suppose, then, a tree 3 metres high, in full vegetation, 
it would decompose all the atmospheric carbonic anhydride in 
its immediate neighbourhood in an hour, or, if it were 36 metres 
high, in a day.’ 
Therefore, in still air and in sunshine, the influence of vegetation 
on the amount of atmospheric carbonic anhydride ought to be very 
perceptible, especially in the neighbourhood of leafy tree crowns 
well above the soil, and consequently not subjected to the imme- 
diate effects of ground air. But wind will play a very important 
part in marking this action. Even, however, on a moderately breezy 
day we think it ought to be possible to trace an action by comparing 
the air at the weather and lee sides, respectively, of a forest or 
large plantation. 
Suppose thisimaginary case: namely, a forest twenty miles long, 
enclosed in a perfectly transparent air-tight, rectangular box, 
36 metres high, open at the weather and lee sides, and so arranged 
that while the tree trunks project through its bottom, no ground 
air can gain admission. Then, with a gentle to moderate breeze 
of force 3-4 on Beaufort’s scale, or velocity of twenty miles to the 
hour, air would remain in contact with the forest for one hour, and 
supposing equal admixture of allits parts, would contain on emerg- 
ing 7/5 less of its original amount of carbonic anhydride—a diminu- 
tion say of 0°25 parts per 10,000 of air, a variation which could 
readily be detected. Of course, on the lee side of a forest under the 
actual conditions nothing like this reduction could be expected, 
owing to vertical movements of the air currents, evolution of 
carbonic anhydride from the soil, &., which may account for 
Ebermayer’s results. 
1 Since writing the above, Horace T. Brown, in his very interesting address as Presi- 
dent of the Chemical Section of the British Association (1899), has described the 
results of experiments on the rate of absorption of atmospheric carbonic anhydride by 
the leaves of growing plants, contained in transparent vessels, through which a current 
of ordinary air passed. In one such experiment the air, before entering the vessel, 
contained 2°80 parts CO2 per 10,000, and on leaving 1-74, showing therefore a loss of 
1:06, which corresponded with a rate of assimilation of 412 c.c. CO2 at N.T.P. per 
square metre of leaf surface per hour. He also found that under constant illumination 
the intake was directly proportional to the tension of the gas. (See ‘‘ Nature,”’ 
60 [1899], p. 479.) 
