their velocity decreases. This is in agreement with observations

on the β rays. In some cases Bragg also observed that the [Greek: alpha] particles are the most efficient ionizers just before they lose their power of ionizing the gas.

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Fig. 43.

Thus we may conclude from these experiments that the [Greek: alpha] particles from a simple radio-active substance traverse a definite distance in air, at a definite pressure and temperature, and that the ionization ends fairly abruptly. If the rays traverse a sheet of metal, the effective range of ionization is diminished by a distance corresponding to ρd, where ρ is the density of the material compared with air and d its thickness. The [Greek: alpha] rays from a thick layer of a simple radio-active substance consist of [Greek: alpha] particles of