A GAS through which a current is parsing may be considered as a mixture of three gases—neutral molecules, electrons, and positive ions. In regions of relatively small field and space charge, each of these gases will show an approximate Maxwellian distribution of energies among the particles, that is, will be in temperature equilibrium within itself, but each gas will have a different temperature. Even at gas pressures so high as a millimetre of mercury, and in an almost field-free space, the temperature of the positive ions will be very much higher than the temperature of the neutral molecules with which they are continually colliding. The only available source of energy of random motion appears to be the electron gas, which is at a still higher temperature. L. H. Thomas (Proc. Royal Soc., A, 121, 464; 1928) derives formulae for the interchange of energy between particles interacting according to the inverse square law and uses them to explain the rapidity with which a Maxwellian distribution of velocities is set up within an electron gas. They may also be used to calculate the temperature of the positive ions from the temperature of electrons and the pressure of the gas in a field-free space.