32 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
These cases were made in several sizes according to requirements, the 
largest being capable of taking a fair-sized leaf of the sunflower. An inlet 
tube a for the air is fixed through one side of the frame, and a correspond- 
ing outlet 6 on the opposite side, the tubes being respectively above and 
below the plane of the leaf. 
The necessary current of air is drawn through the case by a water-pump, 
the volume being determined by carefully-standardized meters B and C 
reading to 20 c.c., and since the volumes of air employed vary from about 
200 to 900 litres or more, errors of measurement are practically non-existent. 
The air on its way from the leaf-cases to the meters has to pass through 
the absorption-apparatus D and E presently to be described, and as this 
- introduces a resistance, the air enters the meters at less than atmospheric 
pressure. This difference of pressure is measured by the mercury mano- 
meters ¢ and @ in connection with the inlet to the meters. During an 
experiment frequent observations are made of these manometers, and. also of 
the barometric pressure and the temperature of the air around the meters, 
the necessary data for reducing the volumes of air to normal temperature 
and pressure being thus obtained. 
In the glass connection between the absorption-apparatus and the meter 
there is a small by-pass so arranged with glass taps that a portion of the air- 
stream can be deflected through small vessels e¢ and 7 containing baryta- 
water. 
These act as “ecuard-tubes,’ and if the baryta-water remains perfectly 
clear throughout the experiment, it is an indication that the absorption of 
carbon dioxide from the air-stream has been complete.* ‘ 
The apparatus used for the absorption of the carbon dioxide from the air- 
stream is similar to the one used by Reiset in his estimations of the carbon 
dioxide of the atmosphere.T 
It is shown in its original form at D in the figure, and consists 
essentially of a long wide glass tube fixed vertically by means of an india- 
rubber cap into a wide-mouthed glass vessel furnished with a second 
opening through which the air enters the apparatus. The height of the tube 
is always 50 cm., but its width may vary according to the amount of air 
which has to be passed in a given time. 
The lower end of the vertical tube, which is adjusted within a few milli- 
* The absorptive power of the Reiset’s apparatus, when due precautions are taken, is so very 
complete that this portion of the apparatus is almost superfluous. 
+ ‘Compt. Rend.,’ vol. 88, p. 1007, and vol. 90, p. 1144; see also Hempel’s “ Gas Analysis,” 
p. 83. 
