92 RESPONSE IN THE LIVING AND NON-LIVING 
completely removed by the adoption of the following 
experimental modification. In the simple arrangement 
for qualitative demonstration of response in metals 
previously described, successive experiments will not 
give results which are strictly comparable (1) unless 
the resistance of the circuit be maintained constant. 
This would necessitate the adoption of some plan for 
keeping the electrolytic contacts at A and B absolutely 
invariable. There should then be no chance of any 
shifting or variation of contact. (2) There must also 
be some means of applying successive stimuli of equal 
intensity. (3) And for certain further experiments 
it will be necessary to have some way of gradually 
increasing or decreasing the stimuli in a definite 
manner. 
Modification of the block method.—By consideration 
of the following experimental modifications of the block 
method (fig. 55), it will be found easy to construct 
a perfected form of apparatus, in which all these con- 
ditions are fully met. The essentials to be kept in 
mind were the introduction of a complete block midway 
in the wire, so that the disturbance cf one half should 
be prevented from reaching the other, and the making 
of a perfect electrolytic contact for the electrodes 
leading to the galvanometer. 
Starting from the simple arrangement previously de- 
scribed where a straight wire is clamped in the middle 
(fig. 55, a), we next arrive at (b). Here the wire 
A Bis placed in a U tube and clamped in the middle 
by a tightly fitting cork. Melted paraffin wax is poured 
to a certain depth in the bend of the tube. The two 
