30 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
information on the “ energetics ” of the leaf, especially as regards its power of 
absorbing and transforming the solar radiation incident upon it. 
Description of Apparatus and Methods. 
In any investigation of the phenomena of assimilation which is based on a 
direct determination of the rate of intake of the carbon dioxide of ordinary 
atmospheric air it 1s evident that, in order to ensure success, the experiments 
must be carried out on a relatively large scale, both as regards the area of 
leaf-surface exposed, and the volume of air passed through the apparatus. 
Another essential condition is that we must have an accurate but not too 
laborious method for determining the amount of carbon dioxide in the air 
both before and after contact with the assimilating leaf. | 
After a considerable amount of preliminary work we adopted the following 
arrangement of apparatus, which has been used throughout this research, and 
has proved well adapted to the purpose for which it was devised. 
The leaf under experiment is enclosed in a flat rectangular case 
(A, fig. 1) consisting of a light wooden frame glazed on two sides. The 
wooden strips before being dove-tailed together to form the frame are care- 
fully dried, and soaked for some time in a bath of hot melted paraffin, 
in order to render the wood impermeable to air. 
The glass plate on one side of the case is permanently fixed, but that 
of the opposite side is moveable, and drops into a rebate in the frame, into 
which it may be fixed air-tight with soft wax-mixture after introducing the 
leaf.* 
In order to maintain the leaf-lamina parallel to the glass sides of the case 
and to ensure sufficient space for the circulation of the air on both sides, the 
leaf rests on threads which are laced through small metal eyes screwed into 
the frame at intervals of about half an inch, and a similar lacing of threads 
above the leaf serves to fix it in the right position. 
If a detached leaf is employed, the petiole passes through an aperture in 
the side of the case, and dips into a small tube outside, which is filled with 
water. If, on the other hand, it is desired to experiment on a leaf which is 
still attached to its plant, there is a slot cut in the wooden frame to about 
half its depth, into which fits a moveable tongue of wood so arranged 
that when the leaf is in position the tongue nearly fills up the slot. The. 
spaces around the leaf-stalk and the tongue are made air-tight with the luting 
mixture already alluded to. 
* The wax luting employed was that recommended by F. F. Blackman, ‘ Phil. Trans.,’ B, 
vol. 186 (1895), p. 522. 
