=41. Ions produced by collision.= The greater part of the radiation from the radio-active bodies consists of a stream of charged particles travelling with great velocity. In this radiation, the α particles, which cause most of the ionization observed in the gas, consist of positively charged bodies projected with a velocity about one-tenth the velocity of light. The β rays consist of negatively charged particles, which are identical with the cathode rays generated in a vacuum tube, and travel with a speed about one-half the velocity of light (chapter IV.). Each of these projected particles, in virtue of its great kinetic energy, sets free a large number of ions by collision with the gas molecules in its path. No definite experimental evidence has yet been obtained of the number of ions produced by a single particle, or of the way in which the ionization varies with the speed, but there is no doubt that each projected body gives rise to many thousand ions in its path before its energy of motion is destroyed. It has already been mentioned (section 29) that at low pressures ions moving under the action of an electric field are able to produce fresh ions by collision with the molecules of the gas. At low pressures the negative ion is identical with the electron set free in a vacuum tube, or emitted by a radio-active substance. The mean free path of the ion is inversely proportional to the pressure of the gas. Consequently, if an ion moves in an electric field, the velocity acquired between collisions increases with diminution of the pressure. Townsend has shown that fresh ions are occasionally produced by collision when the negative ion moves freely between two points differing in potential by 10 volts. If the difference be about _V_ = 20 volts, fresh ions arise at each collision[73]. Now the energy _W_, acquired by an ion of charge _e_ moving freely between two points at a difference of potential _V_, is given by _W_ = _Ve_. Taking _V_ = 20 volts = ²⁰⁄₃₀₀ E. S. units, and _e_ = 3·4 × 10⁻¹⁰, the energy _W_ required in the case of a negative ion to produce an ion by collision is given by _W_ = 2·3 × 10⁻¹¹ ergs. The velocity _u_ acquired by the ion of mass _m_ just before a collision is given by 1 --- _mu²_ = _Ve_, 2 and $$ u = \sqrt{\frac{2Ve}{m}} $$ Now _e_/_m_ = 1·86 × 10⁷ electromagnetic units for the electron at slow speeds (section 82). Taking _V_ = 20 volts, we find that _u_ = 2·7 × 10⁸ cms. per sec. This velocity is very great compared with the velocity of agitation of the molecules of the gas.