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 The emanations possess the property of chemical inertness, and in this respect resemble the gases of the argon family. The emanations are produced in minute amount; but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum. With regard to their rates of diffusion, the emanations of both thorium and radium behave like gases of high molecular weight.

These emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character. These radiations consist entirely of α rays, i.e. particles, projected with great velocity, which carry a positive charge and have a mass about twice that of the hydrogen atom. The emanations do not possess the property of permanently radiating, but the intensity of the radiations diminishes according to an exponential law with the time, falling to half value, from actinium in 4 seconds, from thorium in one minute, and from radium in about four days. The law of decay of activity does not seem to be influenced by any physical or chemical agency.

The emanation particles gradually break up, each particle as it breaks up expelling a charged body. The emanation after it has radiated ceases to exist as such, but is transformed into a new kind of matter, which is deposited on the surface of bodies and gives rise to the phenomena of excited activity. This last property, and the connection of the emanation with it, are discussed in detail in the next chapter.