Page:Collected Physical Papers.djvu/305

Rh The deflection takes place from the modified zero, to which the spot returns during recovery. I give four records (fig. 72): in (a) A is touched with Na$2$CO$3$ (which is an excitant): a permanent current flows from B to A: response to stimulus is in the same direction as the permanent current (positive variation); in (b) A is touched with a trace of oxalic acid (which depresses the excitability), the permanent current is in the same direction as before, but the current of response is in the opposite direction,(negative variation); in (c) A is touched with dilute KHO (3 parts in 1000), the response is exhibited by a positive variation; in (d) A is touched with stronger KHO (3 parts in 100), the response is now exhibited by a negative variation. The last two apparently anomalous results are due to the fact (which will be demonstrated later) that KHO in minute quantities is an excitant, while in larger quantities it is a depressant.