1315 
It is my experience, that well-fixed chambers may remain fit for 
use for 4 weeks, but they should be ventilated regularly between 
the experiments. It is essential, that the air in the hothouse at least 
in the first hours after attaching the chamber is fairly dry, other- 
wise the glue remains soft too long, which may cause the leaf 
to get slightly removed from the chamber. 
Il. The manometer. The two limbs are of unequal diameter, 
therefore the manometer should be tested before use in order to know 
what distance in the narrow tube conforms with a change in 
pressure of 1 cm of water. Fixing the water-level at zero is a very 
simple thing and quickly done, in consequence of the presence of 
a water-reservoir R (attached to the manameter by means of a 
rubber tube with clamp) and of a drain with clamp. Practice teaches 
that hardly any water evaporates from the manometer, so that the 
apparatus can work 48 hours at a stretch, without the water-level 
having to be fixed anew '). | 
UL The electro-magnetic clamp K,. The whole apparatus is really 
based upon the theorem, that the elastic clamps close the rubber 
tubes absolutely and that at the points where the rubber tubes pass 
over the glass tubes, leakage is impossible. What fig. 3 shows for 
Ficus elastica between 5 and 7 p. m. is the best proof for the 
practicability of this theorem. 
About the structure of the clamp itself there is not much more 
to be said, than can be seen in the picture. The electric current 
however should be further discussed. It is yielded by a battery of 
accumulators of 7 cells, therefore with a voltage of 14 Volt and 
the force of the current is for the clamp K, only 1 Ampère. There 
is a very small loss of electricity, for the time used for sucking up 
the water in the manometer is at most 4 seconds. The current of 
the battery is also used for the solenoids of the switch (circuit 
I and II) and for the electromagnetic clamp K, (circuit VI). 
Part of the apparatus being in the hothouse (everything below the 
horizontal line in fig. 1) and the rest in a room of the laboratory 
(switch, recording-apparatus and battery) an amount of connecting 
Wire is necessary. A tube, containing 16 wires, joins the two localities 
and the wires end on terminal boards with numbered terminals, from 
which flexible wires lead towards the different apparatus. Two series- 
switches have been inserted, which can close and break the current 
if necessary botn in the hothouse and in the room. 
1) The U-shaped manometer is therefore also to be preferred in case of the 
non-recording porometer to the ordinary glass tube placed in a vessel of water. 
