The curve exhibits the same peculiarities as those given by Bragg for a thin film of matter of one kind. The ionization of the α particle per unit path increases slowly for about 4 cms. There is then a more rapid increase just before the α particle ceases to ionize the gas, and then a rapid falling off. The ionization does not appear to end so abruptly as is really the case, since there is a correction to be applied for the angle subtended by the cone of rays. The maximum range of the α rays in air was 6·7 cms., a number in agreement with that obtained by Bragg by measurements on the range of the rays from radium.
These results show that the ionization per unit path of the α particle increases at first slowly and then rapidly with decrease of velocity until the rays cease to ionize the gas.
Energy required to produce an ion. From the above results the energy required to produce an ion by collision of the α particle with the gas molecules can readily be deduced. The α particles, emitted from radium itself, are initially projected with a velocity ·88V₀ where V₀ is the initial velocity of projection of the α particles from radium C. The α particles cease to ionize the gas at a velocity ·64V₀. 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 α particle may be expelled with a velocity less than 1·5 × 109 cms. per second and so fail to produce much electrical effect.
These considerations have an important bearing on the question whether matter in general is radio-active. The property of emitting α particles above the critical velocity may well be a property only of a special class of substances, and need not be exhibited by matter in general. At the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of α particles at a rate much greater than that shown by uranium, without producing appreciable electrical or photographic action.
APPENDIX B.
RADIO-ACTIVE MINERALS.
Those natural mineral substances which possess marked radio-active properties have been found to contain either uranium or thorium, one of these elements being always present in sufficient proportion readily to permit its chemical separation and identification by the ordinary analytical methods[[455]].