L1 PROFESSOR B. SANDERSON ON - THE ELECTROMOTIVE 
of the Chemical Society.'" The chamber is provided with a sliding floor of wood, 
covered with glass, which serves to support the apparatus. The wires leading to the 
electrodes, See., enter by openings left on either side of the floor for the purpose. 
Mode of preparing the leaf for observation .—Whether the leaf is on the plant or 
not, it is necessary to fix it in such a way as to render it motionless. In our earlier 
experiments glass forceps were used, ingeniously contrived and constructed by 
Mr. Page, of which the blades were of such form that while one of them applied itself 
to the upper surface of the leaf when in the fully expanded state, the lower supported 
* Fig. 2. 
The Leaf, seen from above. 
The two end plugs and the cross-bar, which stretches from margin to margin, serve to 
prevent it from closing. 
Fig. 3. 
The same Leaf, seen from the side. (Both figures are copied from photographs.) 
the midrib, for the reception of which it was suitably grooved. A figure of this in¬ 
strument was given in our former paper.! We have now adopted a much better 
mode of immobilization which will be understood from the figure. Before otherwise 
interfering with the leaf, plaster of Paris, which has just been mixed with water, is 
introduced between the lobes at either end of the midrib, so as to bridge the space 
between them. A splinter of dry wood is laid across from edge to edge of the lobes, 
the ends of which are cemented by plaster to the marginal hairs. The leaf so prepared 
is unable to close (figs. 2, 3). 
B. The galvanometer used is a Thomson of 5244 ohms resistance. The degree of 
sensibility best adapted for the ordinary purposes of the investigation is such that 
with a thousandth of a Daniell and a resistance of 10,000 ohms, added to that of the 
* Vo], i., 1876, p. 24. 
t Proceedings, 1873, p. 413. 
