PROPERTIES OF THE LEAF OF DION^EA. 
13 
galvanometer, we have a deflection of 350 scale; but for some rheotome observations, 
a much higher sensibility is required. With a view to more accurate reading of small 
deflections, a concave mirror, 10 centims. wide, is placed in front of and below the 
scale, by means of which that part of the graduation nearest to zero can be seen 
enlarged. 
The electrodes.—The most important part of each electrode is a U-sluaped. tube 
containing saturated zinc sulphate, which is supported by a holder of the form shown 
in the figure (fig. 4). Into the end of the U contained in the clamp, sinks a zinc rod, 
Fig. 4. 
A Hon-polarizable Electrode. (From a photograph.) 
which is held by the spring clip, and is thus brought into metallic connexion with, 
the coiled wire. This rod is hammered smooth at the end which is intended to be 
immersed, polished, and amalgamated by treating it sparingly with very weak solution 
of mercury in nitrohydrochloric acid. Into the opposite limb of the (J slips a narrow 
straight tube 1 centim. in length and about 5 millims. in internal width, which is 
charged with paste of kaolin and solution of zinc sulphate. The tube has a rim of 
sealing wax at the top, which prevents it from sinking too far, and is surrounded by a 
conical beak, shaped with the fingers out of modelling clay (kaolin made into a paste 
with 0'75 per cent, solution of chloride of sodium). In order to bring the modelling 
clay to a pencil-like point, a length of several strands of ligature thread steeped in 
the solution is incorporated with it. These electrodes have a resistance of about 
6000 ohms and keep in order for any required time if the atmosphere is saturated 
or nearly so. The construction of the supports for the electrodes can be understood 
from the figure. It is sufficient to notice that the collar slides on the horizontal 
rod of ebonite. This rod can be raised or lowered, or made to rotate round the 
