Page:Collected Physical Papers.djvu/306

286 {|
 * Permanent current.
 * Current of response.
 * A treated with sodium carbonate&bull;
 * $$\leftarrow$$
 * $$\leftarrow$$
 * ,,,,oxalic acid&bull;&bull;
 * $$\leftarrow$$
 * $$\to$$
 * ,,,,very dilute potash&bull;
 * $$\leftarrow$$
 * $$\leftarrow$$
 * ,,,,strong potash&bull;&bull;
 * $$\leftarrow$$
 * $$\to$$
 * }
 * ,,,,strong potash&bull;&bull;
 * $$\leftarrow$$
 * $$\to$$
 * }
 * }
 * }

I will now describe an experiment which will show in a striking manner how exceedingly delicate is the method of electric response to stimulation, and how by its means we can detect and measure traces of physico-chemical changes in different parts of the same solid. Take a wire and touch two points, one with Na$2$CO$3$ solution the other with oxalic acid. Wash the wire. There is no trace left of the previous treatment. Let one contact be permanently made at a normal or previously unacted point N. Let the other exploring contact be moved along from the other end towards N, the wire being mechanically stimulated during the test. The galvanometer spot remains quiescent as long as the exploring contact is over normal areas. But as soon as it touches the zone on which is impressed the invisible image of physico-chemical change, the differential effect of stimulus at once reveals it by producing a vigorous