154 RESPONSE IN THE LIVING AND NON-LIVING 
Let two blocks of paraffin be taken and a large hole 
drilled through both. Next, place a sheet of metal 
between the blocks, and pour melted paraffin round the 
edge to seal up the junction, the two open ends being 
also closed by panes of glass. We shall have then two 
compartments separated by the sheet of metal, and 
these compartments may be filled with water through 
the small apertures at the top (fig. 97, a). 
The two liquid masses in the separated chambers 
thus make perfect electrolytic contacts with the two 
faces A and B of the sheet of metal. 
These two faces may be put in con- 
nection with a galvanometer by 
Y UN Oy Wa BE 
Fic. 97 (b)—Recorp oF RESPONSES OBTAINED 
FROM THE ARovE CELL 



Ten seconds’ exposure to light followed by fifty 
seconds’ recovery in the dark. Thick lines repre- 
sent action in light, dotted lines represent re- 
Fic. 97 (a) covery. 
A, B are the two faces of a 
brominated sheet of sil- 
rer. One face, say A, is lige 
neted on by licht.. The trodes, whose ends dip into the two 
current of response is 
from B to A, across the CHambers. Ifthe sheet of metal have 
plate. 
means of two non-polarisable elec- 
been properly annealed, there will 
now be no difference of potential between the two faces, 
and no current in the galvanometer. If the two faces 
are not molecularly similar, however, there will be a 
current, and the electrical effects to be subsequently 
described will act additively, in an algebraical sense. 
Let one face now be exposed to the stimulus of light. 
A responsive current will be found to flow, from the 
less to the more stimulated face, in some cases, and in 
others in an opposite direction. 
