ation were necessary to prove that this last conclusion was wholly unwarranted; and to-day these laws of the conservation of energy and of matter are more firmly established than ever. The thing that has gone by the board is the old idea of the atoms as the indivisible and irreducible minima of the material universe. For not only do all the radioactive substances give off particles of helium gas positively electrified, but _all bodies, no matter what their composition_, can by suitable treatment, such as exposing them to ultra-violet light, or raising them to incandescence, be made to give off electrons or negatively charged particles, and these electrons are always the same no matter from what kind of substance they come. In a somewhat similar way, we always get positively electrified particles of the mass of the hydrogen atom, or about 1,760 times the mass of the electron, whenever we send an electric charge through a gas at very low pressure, no matter what the kind of gas. Wh
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