162 
Notes . 
Thus by its aid the evolution of C0 2 , by a single germinating seed 
or by a small area of a foliage leaf, can be accurately estimated from 
hour to hour without a break for any desired time, while for the same 
area of leaf the more active absorption of C0 2 in assimilation can be 
easily determined for such short periods of time as fifteen minutes, 
and that at the same time separately for the two surfaces of one and 
the same leaf area. Further, for the purposes of this assimilation, 
a current of air containing any desired proportion of C0 2 , however 
small, can be supplied continuously to the tissue under investigation, 
while, if desired, estimations of the C0 2 evolved in respiration by 
some other part can be carried on simultaneously in a separate cur- 
rent of air freed from C0 2 . This is made possible by the apparatus 
being practically in duplicate; strictly comparative experiments can 
thus be carried out. 
The actual estimation of the C0 2 is accomplished by the well- 
known method of absorption by baryta solution and titration with 
hydrochloric acid. The novelty consists in this— that only a very 
small quantity of baryta solution (under 15 c.c.) is employed in each 
experiment, and that after the absorption the whole of this is titrated 
with acid in the tube in which the absorption has taken place. 
Further, the burettes containing the standard solution are always in 
air-tight connexion with this absorption chamber, and no air beyond 
the current under investigation is ever admitted to the chamber, 
except such as has been carefully freed from C0 2 . The special 
arrangements for stirring and emptying, by means of this air freed 
from C0 2 , and stored under pressure, cannot be entered into here. 
The two currents of air passing continuously through the apparatus 
are generated by two aspirators of a special type, which, worked on 
the principle of Mariotte’s bottle, give a practically constant outflow 
in drops, whatever the level of the water within them, and are 
adapted to work steadily with small rates of flow (50 to 100 c.c. 
per hour). These currents enter the apparatus, either through an 
arrangement for removing the C0 2 when working on respiration 
or when working on assimilation through one for adding C0 2 . 
Both these are so constructed that the current never has to bubble 
through a layer of liquid, and so is supplied to the plant at strictly 
atmospheric pressure, thus avoiding any risk of drawing gases 
mechanically out of the part under investigation. The remover of 
C0 2 is a ‘ tower ’ full of beads, over which a stream of strong potash 
