A beam of pure ${\mathrm{He}}^{++}$ ions was passed through a cell in which the gas pressure could be varied, and the emergent beam was examined for its ${\mathrm{He}}^0$ and ${\mathrm{He}}^{+}$ content. With helium and air in the cell, it was observed that ${\mathrm{He}}^0$ persists to pressures so low that it could not have been formed except by double electron capture by ${\mathrm{He}}^{++}$ in a single collision. In hydrogen the double capture event is much less probable.

The analogous experiment was performed with a helium atomic beam, and at 250 kev and 450 kev in air ${\mathrm{He}}^{++}$, formed by double electron loss, was detected. In hydrogen and helium only upper limits for double electron loss could be established.

These results, with values of the sums (${\sigma }_{20}+{\sigma }_{21}$) and (${\sigma }_{01}+{\sigma }_{02}$) measured in previous researches, made it possible to compute values of ${\sigma }_{20}$ and ${\sigma }_{02}$, as in the following table. Cross sections in units of ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ per atom

A table of the six charge-changing collision cross sections for helium ions is presented. The equations needed to compute the change in the charge composition of a helium ion beam due to passage through a gas film are given.

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