Porometer and a Note on Stomatal Behaviour during Wilting. 49 
of a strip of mica, H, secured by a binding-screw, K, at one end, carrying 
a thin platinum wire, L, which is bent downwards at the free end and poised 
over mercury in a glass tube, M, which connects with the binding-screw N. 
The binding-screws are secured in their respective tubes by means of sealing- 
wax, and IC is separated from the mercury by a sealing-wax plug, P. Leads 
from K and N are carried to a battery and the terminals of a magnetic 
pen, writing on an ordinary clockwork recording drum. We have used 
and found satisfactory a drum supplied by the Cambridge Scientific 
Instrument Company. It is about 13 cm. in diameter and revolves once 
in about twenty-seven minutes, being long enough to record continuously 
for forty hours. 
The mica strip is placed so that the drops from E impinge upon 
its free end. Thus, when a drop falls, the platinum point carried by 
the mica is plunged forcibly into the mercury in M, and by this means 
a circuit is completed through the battery and pen, and the pen is deflected ? 
making a record on the drum. The resilience of the mica immediately 
withdraws the platinum point from the mercury and poises it in readiness 
for the next drop, so that the current passes only momentarily. In order 
that water may not accumulate on the mica and so prevent the strip from 
springing back and breaking the contact, it has been found advisable to 
slope the mica strip so that the water drains from the end. The mercury 
retains a film or drop of water upon its surface, which, however, is no 
disadvantage, since even if the platinum continuously dips into the water, 
the low voltage (about two volts) required to work the pen produces 
very little electrolysis, and at the same time the effects of sparking, which 
occurs when the contact is broken, are lessened. 
It has been found that the mica strip tends to oscillate after a drop has 
fallen, thus making more than one record on the drum. This is not 
a serious drawback because it is readily detected when it occurs ; it is, 
however, easily prevented by raising the recorder till the tube R bears upon 
the mica, thus pressing the platinum point very close to the mercury 
surface. This is found to damp the oscillations and one record per drop is 
obtained. 
The chief difficulty with the apparatus is that when air has entered 
the aspirator bottle, temperature changes cause changes in the volume and 
a consequent alteration in the rate of dropping. To overcome this, it has 
"been found necessary to place the aspirator in a water bath at a constant 
temperature. An electrically heated bath, constant to +0-02° C., has 
been used and found extremely satisfactory, but such accuracy as this 
is not essential. It is obvious that a greater volume of air in the bottle 
entails less accuracy, so that an experiment should be started with the 
bottle full of water, and refilling should be resorted to as frequently as 
possible. 
E 
