nuously increasing E. M. F. from zero to a maximum, and then (2) to a continuously diminishing E. M. F. from the maximum back to zero. The required information may be obtained by the interpretation of the characteristic curve, in which the abscissæ represent the impressed E. M. F., and the ordinates give the corresponding values of the current. A continuously increasing E. M. F. can be made to act on the sensitive substance, by the movement of a slider over a potentiometer wire, say, to the right. The scale readings of the potentiometer give the values of the E. M. F. The readings of the galvanometer in circuit give the corresponding values of the current; movement of the slider to the left produces a continuous diminution of E. M. F. The characteristic curve can thus be obtained from the observed values of E. M. F. and the corresponding currents.
This is a bare outline of the principle of the investigation described in this Paper. Various experimental modifications have to be introduced in order to overcome certain difficulties and render the method practical.
Before entering upon the subject of experimental arrangements, I may repeat what I pointed out in my paper on Electric Touch that in regard to their response to electric radiation, there are three types of substances, positive, negative and neutral, differentiated "by the characteristic curves of variation of current with E. M. F." The positive exhibits an increase of conductivity; the negative shows a diminution of conductivity: and the neutral does not exhibit any conductivity change. The first two of these classes again fall into two subdivisions, of substances which exhibit self-recovery and those which do not. The characteristic
