to this may doubtless be ascribed the fact that they have remained so long undetected.
I was then led to ask myself whether bodies which are themselves in a state of strained internal equilibrium would not emit "N" rays. That they do so is indeed confirmed by experiment. Rupert's drops, hardened steel, hammer-hardened brass, melted sulphur of crystalline structure, etc., are spontaneous and permanent sources of "N" rays. One can, for instance, repeat the experiments with the clock dial, employing, instead of a compressed body, a hardened steel tool, such as a chisel or file, or even a pocket-knife, without in any way bending or compressing them; similarly, bringing near to a small mass of phosphorescent calcium sulphide a knife-blade or bit of unannealed glass is sufficient to increase the phosphorescence. Non-hardened steel is without action; a chisel which is successively hardened and softened in turn is active when hard and inactive when the temper is taken out of it. These actions traverse, without any notable weakening, a plate of aluminium 1.5 cm. thick, an oak board 3 cms. thick, black paper, etc.
