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 to this internal activity of material bodies, to the pursuit of stability. Wiedemann, Warburg, Tomlinson, MM. Duguet, Brillouin, Duhem, and Bouasse have revived the old experimental researches of Coulomb and Wertheim on the elasticity of bodies, the effects of pressures and thrusts, the hammering, tempering, and annealing of metals.

The internal activity manifested under these circumstances presents quite remarkable characteristics which cannot but be compared to the analogous phenomena presented by living bodies. Thus have arisen even in physics, a figurative terminology, and metaphorical expressions borrowed from biology.

Comparison of the Activity of Particles with Vital Activity.—Since Lord Kelvin first spoke of the fatigue of metals, or the fatigue of elasticity, Bose has shown in these same bodies the fatigue of electrical contact. The term accommodation has been employed in the study of torsion, and according to Tomlinson for the very phenomena which are the inverse of those of fatigue. The phenomena presented by glass when acted on by an external force which slowly bends it, have been called facts of adaptation. The manner in which a bar of steel resists wire-drawing has been compared to defensive processes against threatened rupture. And M. C. E. Guillaume speaks somewhere of "the heroic resistance of the bar of nickel-steel." The term "defence" has also been applied to the behaviour of chloride or iodide of silver when exposed to light.

There has been no hesitation in using the term "memory" concurrently with that of hysteresis to designate the behaviour of bodies acted on by magnetism or by certain mechanical forces. It is