Page:Popular Science Monthly Volume 71.djvu/533

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RADIOACTIVITY
527

The Actinium Group

The Radium Group

In connection with these tables it may be well to consider the a particles a little more. It has been found that the particles of any one product are emitted with the same velocity. It is found that after passing through a definite distance of gas, they then cease to ionize it. If the gas is air under normal conditions of pressure and temperature, this distance will be the range of the particle. All experiments up to the present indicate that the α particles of the different products differ only in the speed of projection, this speed determining the range of ionization. Rutherford has found an empirical relation between the range of the particle and its velocity at any point in its path. If r is the remaining range after passing through a screen, its velocity is , where is the initial velocity of the particles emitted from radium C, and is cm. per sec. The initial velocity of expulsion of an particle from a certain product will then be a constant. The value of e/m for all rays measured has been found to be the same and to be about electro-magnetic units. It thus follows that all the radio-elements possess the