18 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
and IV., as the case may be. When longer periods have to be investigated the 
rheotome previously described by me, in relation to my experiments on the time 
relations of the excitatory process in the heart, answers perfectly. 
D. Methods of Excitation .—For electrical excitation (whether by the voltaic or 
induction current) it is necessary to use non-polarizable electrodes. As the space 
between the upper surfaces of the lobes is insufficient to allow of the introduction of 
two electrodes of the form used for leading off, it is convenient to use a simpler form 
which, although not so perfect, answers the purpose completely. It is merely a zinc 
rod of which the amalgamated end is sheathed in wash-leather steeped in solution of 
zinc sulphate. The sheathed end is enclosed in a larger sheath of the same material, 
steeped in solution of salt, which ends in a point convenient for contact. The zinc 
rods are held by supports which allow of their points being brought into any required 
position with accuracy. 
Diagram of the connexions of the Pendulum Rheotome as used for determining time of 
commencement of first phase, rate of propagation, &c. 
PART III. 
Electromotive Properties of the Uninjured Leaf. 
The preliminary examination which was made in 1876 of the electromotive properties 
of the uninjured leaf appeared to show, first, that the external surface of each lobe 
is always positive to the internal surface, and, secondly, that in general the part of the 
midrib which is nearest to the sensitive hairs is positive to other parts of the external 
surface of the leaf. The more complete knowledge which has been acquired by work 
done since, justifies the statement that these two propositions express the truth, 
