Page:Popular Science Monthly Volume 71.djvu/533

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 $\alpha$ 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 $\alpha$ 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 $\alpha$ particle and its velocity at any point in its path. If r is the remaining range after passing through a screen, its velocity is $V=.348\ V_{0}\ {\sqrt {r}}+1.25$, where $V_{0}$ is the initial velocity of the $\alpha$ particles emitted from radium C, and is $2.06\ 10^{9}$ cm. per sec. The initial velocity of expulsion of an $\alpha$ particle from a certain product will then be a constant. The value of e/m for all $\alpha$ rays measured has been found to be the same and to be about $5.07\ 10^{3}$ electro-magnetic units. It thus follows that all the radio-elements possess the