Page:Radio-activity.djvu/574

 where V_{0} is the initial velocity of projection of the α particles from radium C. The α particles cease to ionize the gas at a velocity ·64V_{0}. From this it can at once be deduced that ·48 of the total energy of the α particle, shot out by radium itself, is absorbed when it ceases to ionize the gas. Assuming that the heating effect of radium at its minimum activity—25 gram calories per hour per gram—is a measure of the kinetic energy of the expelled α particles, it can be calculated that the kinetic energy of each α particle is 4·7 × 10^{-6} ergs. The amount of energy absorbed when the α particle just ceases to ionize the gas is 2·3 × 10^{-6} ergs. Assuming that this energy is used up in ionization, and remembering that the α particle from radium itself produces 86000 ions in its path (section 252), the average energy required to produce an ion is 2·7 × 10^{-11} ergs. This is equivalent to the energy acquired by an ion moving freely between two points differing in potential by 24 volts.

Townsend found that fresh ions were produced by an electron for a corresponding difference of potential of 10 volts. Stark, from other data, obtained a value 45 volts, while Langevin considers that 60 volts is an average value. The value obtained by Rutherford and McClung for ionization by X-rays was 175 volts, and is probably too high.

Rayless changes. We have seen that the α particles from the radio-active substances are projected with an average velocity not more than 30 per cent. greater than the minimum velocity, below which the α particles are unable to produce any ionizing, photographic, or phosphorescent action. Such a conclusion suggests that the property of the radio-active substances of emitting α particles has been detected because the α particles were projected slightly above this minimum velocity. A similar disintegration of matter may be taking place in other substances at a rate much greater than in uranium without producing much electrical effect, provided the α particles are projected below the critical velocity.

The α particle, on an average, produces about 100,000 ions in the gas before it is absorbed, so that the electrical effect observed is about 100,000 times as great as that due to the charge carried by the α particles alone.

It is not unlikely that the numerous rayless products which have been observed may undergo disintegration of a similar character to the products which obviously emit α rays. In the rayless product the