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This method is specially suited for studying the effect of electric radiation on discontinuous particles. Since the action of radiation is one of surface, the larger is the superficial area the greater is the result; it is evident that loose particles expose a large surface to the incident radiation. Moreover, as the effective total resistance of the mass of particles is due to resistances of surface contacts, any change induced in the surface layers will cause great variation of the total resistance. In a continuous solid, on the other hand, only a comparatively thin molecular layer on the surface is acted upon; but this has little effect on the conductivity of the mass in the interior, protected by the outer conducting sheet. A slight conductivity variation can, however, be detected if the continuous solid takes the form of an extremely thin layer. I shall presently show that for the detection of molecular strain in a continuous solid the electromotive variation is the more suitable.

It has been stated that in the positive class there is produced a diminution, and in the negative an increase of resistance. These opposite reactions seem at first difficult to understand, but about their reality there can be no doubt. In another paper to be shortly communicated, I shall give an account of an independent inquiry in which the positive, the neutral, and the negative classes of substances are differentiated by their characteristic curves. From the strongly positive substance like Fe, Mg, to the pronounced negative like K, there are numerous gradations. In silver we have a