A positive ray apparatus was used to investigate the products of ionization by electron impact in mixtures of helium, neon, and argon. The variation with pressure of the ratio of the two types of positive ions present was investigated in detail for three cases. Case 1: A mixture of half helium and half neon was investigated up to 0.15 mm pressure. The ratio ${\mathrm{He}}^{+}$/${\mathrm{Ne}}^{+}$ was found to decrease regularly between 0.03 mm and 0.15 mm. At 0.03 mm the mean free path is approximately equal to the dimensions of the apparatus. The suggested reaction is: Ne+${\mathrm{He}}^{+}$→${\mathrm{Ne}}^{+}$+He. Case 2: A mixture of half neon and half argon was investigated throughout the same pressure range. The ratio ${\mathrm{Ne}}^{+}$/${\mathrm{A}}^{+}$ decreased regularly between 0.05 mm and 0.15 mm, but this decrease was less rapid than that of the ratio in Case 1. The suggested reaction is: A+${\mathrm{Ne}}^{+}$→${\mathrm{A}}^{+}$+Ne. Case 3: This mixture was half helium and half argon, and the pressure range was the same as in the first two cases. The ratio ${\mathrm{He}}^{+}$/${\mathrm{A}}^{+}$ remained constant within the limits of experimental error. Case 3a: A mixture of 15 percent helium and 85 percent argon was also investigated as there were theoretical grounds for believing that the rate of variation of ${\mathrm{He}}^{+}$/${\mathrm{A}}^{+}$ with pressure would be greater in such a mixture. In that case the ratio ${\mathrm{He}}^{+}$/${\mathrm{A}}^{+}$ was found to decrease slightly. The suggested reaction is: A+${\mathrm{He}}^{+}$→${\mathrm{A}}^{+}$+He.

The observed effects are best explained by a type of collision of the second kind which is equivalent to ionization by positive ions. To account for the results obtained an electron must be transferred from an atom to an ion at a certain fraction of the collisions between an atom and an ion of higher ionizing potential. The results obtained at low pressures corroborated the values for the probability of ionization in these gases obtained by K. T. Compton and C. C. Van Voorhis.

• Received 5 February 1927

DOI:https://doi.org/10.1103/PhysRev.29.683