accessible to radiation in loose particles is very much enlarged. Moreover, the resistance offered to the particles is not due to the individual solid lumps, but to the resistance of surface layer. It is precisely the surface layers that are affected by radiation, and hence the marked variation of resistance.
When the particles become continuous, the radiation can only affect the extremely thin layer of molecules on the surface, the mass in the interior being shielded by the outer conducting sheet; the molecular changes produced on the surface layer do not affect to any appreciable extent the conductivity of the mass.
For detection of strain effect in continuous solids the method of electromotive variation is the more suitable. It has been shown that light causes a P. D. between the acted and unacted plates. I shall next deal with the question whether mechanical stimulation gives rise to an electromotive variation between the acted and unacted plates.
1. The Strain Cell
For the purpose of the experiment, I made a voltaic element composed of two pieces WW′, taken from the same metal wire. These are fixed parallel to each other in an L-shaped piece of ebonite (fig. 42). The wires at their lower ends are fixed to the ebonite piece by means of ebonite screws SS′. The upper ends are fixed to metallic rods EE′ (which also serve as the electrodes), kept moderately stretched by springs CC′. The two electrodes lead to a sensitive dead-beat galvanometer of D'Arsonval type. A long handle, A, provided with a pointer, could be attached either to E or E′, and by its means either of the wires could
