The range or distance traversed in a gas at ordinary pressures is a few centimetres.  The following table, compiled by Geiger, gives the range in air at the temperature of 15 deg.  C.: 

cms.                   cms.                   cms. 
Uranium 1 —   2.50    Thorium —     2.72     Radioactinium  4.60
Uranium 2 —   2.90    Radiothorium  3.87     Actinium X —   4.40
Ionium —      3.00    Thorium X —   4.30     Act Emanation  5.70
Radium —      3.30    Th Emanation  5.00     Actinium A —   6.50
Ra Emanation  4.16    Thorium A —   5.70     Actinium C —   5.40
Radium A —    4.75    Thorium C1 —  4.80
Radium C —    6.94    Thorium C2 —  8.60
Radium F —    3.77

It will be seen that the ray of greatest range is that proceeding from thorium C2, which reaches a distance of 8.6 cms.  In the uranium family the fastest ray is

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that of radium C. It attains 6.94 cms.  There is thus an appreciable difference between the ultimate distances traversed by the most energetic rays of the two families.  The shortest ranges are those of uranium 1 and 2.

The ionisation effected by these rays is by no means uniform along the path of the ray.  By examining the conductivity of the gas at different points along the path of the ray, the ionisation at these points may be determined.  At the limits of the range the ionisation

{Fig. 13}

ceases.  In this manner the range is, in fact, determined.  The dotted curve (Fig. 13) depicts the recent investigation of the ionisation effected by a sheaf of parallel rays of radium C in air, as determined by Geiger.  The range is laid out horizontally in centimetres.  The numbers of ions are laid out vertically.  The remarkable nature of the results will be at once apparent.  We should have expected that the ray at the beginning of its path, when its velocity and kinetic energy were greatest, would have been more effective than towards the end of its range

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when its energy had almost run out.  But the curve shows that it is just the other way.  The lagging ray, about to resign its ionising properties, becomes a much more efficient ioniser than it was at first.  The maximum efficiency is, however, in the case of a bundle of parallel rays, not quite at the end of the range, but about half a centimetre from it.  The increase to the maximum is rapid, the fall from the maximum to nothing is much more rapid.

It can be shown that the ionisation effected anywhere along the path of the ray is inversely proportional to the velocity of the ray at that point.  But this evidently does not apply to the last 5 or 10 mms. of the range where the rate of ionisation and of the speed of the ray change most rapidly.  To what are the changing properties of the rays near the end of their path to be ascribed?  It is only recently that this matter has been elucidated.