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very firmly by means of the screws into the tap, whilst the latter, 
when well greased, will remain easy to turn. As was said before, 
each of the three taps is provided with such a clip. 
To put the apparatus into operation first of all the necessary 
amount of mercury must be brought into the reservoir |, the india- 
rubber tube 2 and the U-shaped tube c, after which the remaining 
part is filled with water. Then the taps have to be tried, in a way 
I shall not describe here in details, whether they close well, and 
finally the plant is connected with the apparatus in the manner 
shown in fig, A. 
It goes without saying that during the experiment one can measure 
as often as one likes the water driven into the plant during a cer- 
tain time. To do so tap d is closed and the pressure for a moment 
neutralized, to which in most cases there will be no objection. The 
mercury is drawn off and replaced by water, when for a moment 
the taps h and k are opened simultaneously. The experiment can be 
immediately continued by causing the pressure to operate again. 
In order to make the tapping of the mercury possible the tube 
leading off the mercury may not be too narrow at m and its lumen 
must at’ least amount to 3 mM. The drawn off mercury can now 
and again be poured into the reservoir 1, so that for lengthy expe- 
riments a limited amount of mercury is an 
The size of the drops overflowing at € the edge of the U-shaped 
tube depends on the width of the tube and can thus be regulated 
within certain limits according to special wants. In the apparatus 
I am using at present, the top of the U-shaped tube measured on the 
outside has a thickness of about 2mM. The size of the drops, which 
is very constant, amounts to 0.02 cM*. So the apparatus enables 
the experimentist to make very accurate observations. Moreover in 
such cases when one wishes to take measurements at not too long 
intervals without neutralizing the pressure, one can easily attain 
one’s aim by counting the drops flowing over. 
The apparatus has been made according to my directions by the 
firm d. C. To. Martius at Utrecht and is brought by the same 
into the trade. 
(November 21, 1901). 
in. 
