When the alpha ray has sufficiently slowed down, its power of passing right through atoms, without appreciably experiencing any effects from them, diminishes.  The opposing atoms begin to exert an influence on the path of the ray, deflecting it a little.  The heavier atoms will deflect it most.  This effect has been very successfully investigated by Geiger.  It is known as “scattering.”  The angle of scattering increases rapidly with the decrease of velocity.  Now the effect of the scattering will be to cause some of the rays to complete their ranges

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or, more accurately, to leave their direct line of advance a little sooner than others.  In the beautiful experiments of C. T. R. Wilson we are enabled to obtain ocular demonstration of the scattering.  The photograph (Fig. 14.), which I owe to the kindness of Mr. Wilson, shows the deflection of the ray towards the end of its path.  In

{Fig. 14}

this case the path of the ray has been rendered visible by the condensation of water particles under the influence of the ionisation; the atmosphere in which the ray travels being in a state of supersaturation with water vapour at the instant of the passage of the ray.  It is evident that if we were observing the ionisation along a sheaf of parallel rays, all starting with equal velocity,

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the effect of the bending of some of the rays near the end of their range must be to cause a decrease in the aggregate ionisation near the very end of the ultimate range.  For, in fact, some of the rays complete their work of ionising at points in the gas before the end is reached.  This is the cause, or at least an important contributory cause, of the decline in the ionisation near the end of the range, when the effects of a bundle of rays are being observed.  The explanation does not suggest that the ionising power of any one ray is actually diminished before it finally ceases to be an alpha ray.

The full line in Fig. 13 gives the ionisation curve which it may be expected would be struck out by a single alpha ray.  In it the ionisation goes on increasing till it abruptly ceases altogether, with the entire loss of the initial kinetic energy of the particle.

A highly remarkable fact was found out by Bragg.  The effect of the atom traversed by the ray in checking the velocity of the ray is independent of the physical and chemical condition of the atom.  He measured the “stopping power” of a medium by the distance the ray can penetrate into it compared with the distance to which it can penetrate in air.  The less the ratio the greater is the stopping power.  The stopping power of a substance is proportional to the square root of its atomic weight.  The stopping power of an atom is not altered if it is in chemical union with another atom.  The atomic weight is the one quality of importance.  The physical

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state, whether the element is in the solid, liquid or gaseous state, is unimportant.  And when we deal with molecules the stopping power is simply proportional to the sum of the square roots of the atomic weights of the atoms entering into the molecule.  This is the “additive law,” and it obviously enables us to calculate what the range in any substance of known chemical composition and density will be, compared with the range in air.