| Sign of electric touch. | Action of radiation on the fresh or A variety of the substance. | Further action of radiation on the radiation product or B variety. |
| Positive, e .g., iron …… | Diminution of resistance. | Increase of resistance. |
| Negative, e.g., arsenic … | Increase of resistance. | Diminution of resistance. |
We have thus two distinct classes of phenomena dependent on the sign of electric touch. If KA represents the conductivity of the fresh substance, and KB the conductivity of the radiation product, then
(1) With positive substances, as the conductivity of the radiation product is greater (KB>KA)the first action of radiation would be to produce a diminution of resistance. This diminution will continue to be exhibited till the maximum amount of B variety is produced. The further action of radiation now will be to reconvert B into A; but as KA<KB there would now be produced a diminution of conductivity, and a galvanometer in circuit will indicate an electrical reversal. The reconverted A variety may again be transformed to a greater or less extent to B, and in this way a series of reversals may take place, due to the continued action of radiation producing oscillation in molecular or atomic groupings. I shall designate this as the phenomenon of radio-molecular oscillation.
(2) With negative substances the conductivity of the radiation product is less (K < KA), and the first action of radiation will therefore be an increase of resistance. The phenomena exhibited by these negative substances will precisely be opposite to those shown by the positive substances.
The above is but an approximate representation of the phenomena. To be more accurate, one has to take into account the partial changes and the effect of radiation on these changed products. Thus, at first suppose the substance to be entirely made up of A variety (this would rarely be the case). The first flash of radiation converts a large portion of A into B, the substance now being a mixture of A and B. The action of the next flash would be to convert the unchanged A into B, and reconvert to a more or less extent B into A. The electric response will thus be very strong at the beginning, but will become continuously less and less. When the proportion of B has attained a maximum value, the reconversion of B into A will become relatively large, and thus give rise to reversal effect.
I spoke of the conversion "to greater or less extent" of one variety into the other. There is also the question of the relative stability of the two varieties under the given conditions. From the above it will be seen what possibilities there are in the way of different combinations, and the varied phenomena thereby rendered possible. I will presently describe some of the typical cases.
