Ions produced in hydrogen by electron impact.-Using an apparatus previously described in which positive ions formed by impact of electrons of definite energy (V1+V2) are accelerated and then deflected magnetically around a semi-circle into a Faraday cylinder, the relative numbers of ions of types H+, H+2 and H+3 were measured as a function of pressure from <0.1×10-4 mm to.006 mm, and also as a function of impact energy (V1+V2) to 60 volts. At low pressures only H+2 ions are formed; as the pressure is increased the percentage of H+3 increases in proportion to the pressure. In the apparatus used, the percentage of H+ increased with pressure but did not exceed 4 percent, while the percentage of H+3 ions reached 60. These results confirm the conclusions of Dempster and Smyth that the primary process in the ionization of hydrogen is the ionization of the molecule without dissociation. The previously measured ionization potential at 16 volts (confirmed in this work) is that for the formation of H+2. H+3 also appears at this potential, but as a result of a secondary process. It appears that the H+2 ion is readily dissociated by collision and that the H+ ion formed may unite with the H2 molecule collided with or with some other molecule to form H+3. The interpretation of ionization potentials reported by other observers is discussed in the light of these results. Ions produced in helium containing hydrogen, by electron impact.-The percentage of H+ ions found was greater even than the percentage of H+2, while no H+3 ions were observed. Evidently the primary ions H+2 are readily dissociated by impact with He atoms. Evidence was found for the ions HeH+ and also for an ion with me=6, perhaps HeH+2.