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. 

 Tliese two faces may be put in con- 

 nection with a galvanometer by 



s \~..r / r ~..s . 



FIG. 97 (a) 



FIG. 97 (6). KECORD OF RESPONSES OBTAINED 

 FROM THE ABOVE 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- 

 covery. 



A, B are the two faces of a 

 brominated sheet of sil- 

 ver. One face, say A, is 

 acted on by light. The 

 current of response is 

 from B to A, across the 

 plate. 



means of two non-polarisable elec- 

 trodes, whose ends dip into the two 

 chambers . If the sheet of metal have 

 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. 



