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Botany. — “On the Hydrosimeter”. By Prof. J. W. Mott. 
Many years ago I published the results of an investigation on 
the exudation of drops and injection of leaves 1), The greater part of this 
investigation was made with cut branches into which water was 
driven by means of pressure of mercury. The instrument used for this 
purpose was as simple as possible and consistedof nothing but a U-shaped 
tube with a short and a long leg. The branch was fixed on the 
short leg of the tube filled with water, after which mercury was 
poured into the other leg until the desired pressure was obtained. 
This apparatus was rather imperfect, the pressure rapidly changing 
on account of the falling of the mercury in the long leg and the 
rising in the short one, and never remaining constant for a moment; 
so that in the experiments taken with this apparatus after some 
hours the pressure was generally for the greater part or entirely 
neutralized. The measuring of the quantity of water driven in could 
take place but imperfectly; it would have given rather much dif- 
ficulty to arrive at a greater precision. 
In the meanwhile the apparatus was sufficient for the purpose I 
then had in view. Yet the question arose in my mind whether it 
would not be possible to construct a better one, with which the 
driving in of the water could take place under constant pressure, 
whilst at the same time there would be a possibility of measuring 
accurately at any moment of the experiment the quantity of water 
driven in. Such an apparatus being of use for various botanical 
investigations, I have thought it worth while to solve this problem, 
and in this manner the apparatus has been made which I will now 
describe under the name of Hydrosimeter *). 
In this apparatus constant pressure has been obtained in the 
following manner. 
The mercury causing the pressure is in a flask of MARIoTTsE, to 
which an india-ruber tube is attached, so that it can be placed 
higher and lower. In this way a constant higher level is obtained. 
This tube conveys the mercury finally into a U-shaped tube, placed 
upright with the curve downwards and sealed into in a glass- 
reservoir filled with water, to which also the plant under observation 
has been attached. In this manner the constant lower level is 
1) Untersuchungen über Tropfenausscheidung und Injection bei Blättern. Proc. and 
“Communie. of the Royal Acad. of Sciences, Sect. Physics, 2nd Series, Vol. XV. 
2) From Sep = water and aorg = thrust, so gauge of waterpressure. 
16 
Proceedings Royal Acad. Amsterdam. Vol. IV, 
