of +90. The reversals followed each other almost regularly, till the substance became insensitive.
III. Lastly, there may be a class of substances where the conversion from one state to the other is not complete. Here, again, we get two sub-divisions, owing to the distinction between positive and negative substances.
Taking first the case of a positive substance (see fig. 26 (β)), the original conductivity of which is represented by a; the action of the first few flashes of radiation would be to produce a great increase of conductivity by the formation of B variety; the next flashes
Fig. 26. "Damped" Molecular Oscillation.
convert B back to A, but not completely, and the negative deflection will be less than the previous positive deflection. Owing to this "damping" effect, the oscillation curve will approximate to a logarithmic decrement curve. After a series of reversals the oscillation dies away, and the substance becomes almost inert. A glance at the hypothetical curve β shows that at the inert stage, b, the substance as a whole is more conducting than in the fresh condition a (see figs. 27, 28).
The opposite should be the case with negative substances (fig. 26α).
