50 Laidlaw and Knight. — A Description of a Recording 
Recently boiled distilled water is used in the aspirator in order to avoid 
the inconvenient accumulation of air bubbles in the tubes of the apparatus, 
when the temperature of the water is raised to that of the bath. 
A record obtained with this apparatus consists of a series of marks 
upon the clockwork drum, the distance between two adjacent ones repre- 
senting the time taken for a drop to be formed and discharged. The 
distances between adjacent marks will therefore be a measure of the rate 
at which air is being drawn into, and water out of, the aspirator bottle if the 
drops are all of the same size. Tests have been carried out to determine 
the weight of water per drop. Successive single drops were found to vary 
considerably less than i per cent., and this variation includes errors of 
collecting and weighing. Variation in the rate of dropping produces 
variation in the size of the drops — a quicker rate giving larger drops, but this 
variation is most marked with quick rates. Over the range which has been 
found convenient in experiment, i. e. not quicker than two or three drops 
per minute, the variation in size of the drops was again less than i per cent. 
In addition to the change in size of the drops, error may result from 
a change of temperature of the air in the glass or rubber connexions which 
are not in the water bath, namely the connexion from the leaf-chamber 
to the air intake-tube of the aspirator bottle. Such a temperature change 
would result in a change of volume, and consequently of the rate of drop- 
ping. To reduce this error to a minimum it is advisable to reduce the 
volume of air in these connexions to a minimum, by using narrow-bore 
thick-walled rubber and glass tubing, and to shorten the distance between 
plant and aspirator as far as possible. If the whole apparatus is shaded 
from direct sunlight sudden temperature changes will be avoided. A water 
jacket for these connexions has been contemplated, but it has not been found 
necessary to resort to this. 
Before employing the apparatus for investigation of stomatal changes, 
many preliminary experiments were carried out, using fine capillary tubes 
and platinum discs with small punctures in them, in order to estimate the 
accuracy of the apparatus under constant conditions. No purpose would 
be served by a full account of these preliminary investigations, but the 
conclusions drawn from them will be briefly outlined. 
Several types of air intake-tube were tried, the air being made to bubble 
from apertures of various sizes and shapes, but a square-cut end was found 
to be as satisfactory as any form. Similarly a square-cut dropping-tube 
(E in Fig. i) was found satisfactory. 
Slow rates of dropping tend to decrease the accuracy, possibly due 
to the fact that when a drop remains for some time attached to the dropping- 
tube there is a greater possibility of its being prematurely shaken off, 
and also greater opportunity for temperature changes to become effective. 
The most convenient and at the same time accurate rates are from one drop 
