38 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
corresponding to from 0°1 to 0°15 of a cubic centimetre of carbon dioxide. This 
corresponds approximately to the carbon dioxide in from 350 to 500 «ae. of 
air, and since not less than 400 litres of air is passed through the absorption- 
apparatus for every 100 c.c. of soda-solution, the percentage-error of titration 
on the amount of carbon dioxide estimated is not more than 0°08 to 0°12 per 
cent. of the true amount. 
Determination of the Leaf-Avrea. 
The most convenient plan for determining leaf-areas with exactness is to 
place the leaves at the close of an experiment in contact with sensitized 
paper in an ordinary photographic printing-frame and expose them to 
light for a short time. The outline of the leaf-print is then followed with an 
Amsler’s planimeter set to read off square centimetres. This is a far more 
accurate and rapid method than that of cutting out a facsimile of the leaf 
from paper of a known weight per unit area. 
Apparatus for Increasing the Amount of Carbon Dioxmde in Air. 
If it is desired to determine the assimilative power of a leaf in an atmo- 
sphere artificially enriched with carbon dioxide, we employ a form of 
apparatus which is a modification of the generator described by Blackman. * 
The air-stream before entering the leaf-case is passed through a small tower 
containing fragments of marble over which is slowly dropped very dilute 
acid at such a rate as to give approximately the desired amount of carbon 
dioxide to the air current. Knowing the strength of the acid employed and 
the volume of air passed, it is easy to adjust the rate of flow of the acid 
approximately to the desired point. 
The stream of air as it leaves this apparatus is divided, one part of the split 
current passing through the experimental leaf-case, whilst the other passes 
direct to a Reiset’s absorption apparatus and separate meter for the exact 
estimation of the carbon dioxide it contains. 
Rotating Sectors. 
In those cases where it is desired to vary at will, and in a known ratio, the 
amount of radiant energy falling on a leaf, it is necessary to use some kind of 
screen which will arrest equal proportions of all the undulations of varying 
wave-length ; in other words the screen must exercise no “ selective absorption” 
on the radiations. 
* ¢ Phil. Trans.,’ vol. 186 (1895), p. 495. 
