Part 16 (1/2)
In spite of uncertainties which are not yet entirely removed, it cannot be denied that many experiments render it probable that in radioactive bodies we find ourselves witnessing veritable transformations of matter.
Professor Rutherford, Professor Soddy, and several other physicists, have come to regard these phenomena in the following way. A radioactive body is composed of atoms which have little stability, and are able to detach themselves spontaneously from the parent substance, and at the same time to divide themselves into two essential component parts, the negative electron and its residue the positive ion. The first-named const.i.tutes the beta, and the second the alpha rays.
The emanation is certainly composed of alpha ions with a few molecules agglomerated round them. Professor Rutherford has, in fact, demonstrated that the emanation is charged with positive electricity; and this emanation may, in turn, be destroyed by giving birth to new bodies.
After the loss of the atoms which are carried off by the radiation, the remainder of the body acquires new properties, but it may still be radioactive, and again lose atoms. The various stages that we meet with in the evolution of the radioactive substance or of its emanation, correspond to the various degrees of atomic disaggregation.
Professors Rutherford and Soddy have described them clearly in the case of uranium and radium. As regards thorium the results are less satisfactory. The evolution should continue until a stable atomic condition is finally reached, which, because of this stability, is no longer radioactive. Thus, for instance, radium would finally be transformed into helium.[40]
[Footnote 40: This opinion, no doubt formed when Sir William Ramsay's discovery of the formation of helium from the radium emanation was first made known, is now less tenable. The latest theory is that the alpha particle is in fact an atom of helium, and that the final transformation product of radium and the other radioactive substances is lead. Cf. Rutherford, op. cit. pa.s.sim.--ED.]
It is possible, by considerations a.n.a.logous to those set forth above in other cases, to arrive at an idea of the total number of particles per second expelled by one gramme of radium; Professor Rutherford in his most recent evaluation finds that this number approaches 2.5 x 10^{11}.[41] By calculating from the atomic weight the number of atoms probably contained in this gramme of radium, and supposing each particle liberated to correspond to the destruction of one atom, it is found that one half of the radium should disappear in 1280 years;[42]
and from this we may conceive that it has not yet been possible to discover any sensible loss of weight. Sir W. Ramsay and Professor Soddy attained a like result by endeavouring to estimate the ma.s.s of the emanation by the quant.i.ty of helium produced.
[Footnote 41: See _Radioactive Transformations_ (p. 251). Professor Rutherford says that ”each of the alpha ray products present in one gram of radium product (_sic_) expels 6.2 x 10^{10} alpha particles per second.” He also remarks on ”the experimental difficulty of accurately determining the number of alpha particles expelled from radium per second.”--ED.]
[Footnote 42: See Rutherford, op. cit. p. 150.--ED.]
If radium transforms itself in such a way that its activity does not persist throughout the ages, it loses little by little the provision of energy it had in the beginning, and its properties furnish no valid argument to oppose to the principle of the conservation of energy. To put everything right, we have only to recognise that radium possessed in the potential state at its formation a finite quant.i.ty of energy which is consumed little by little. In the same manner, a chemical system composed, for instance, of zinc and sulphuric acid, also contains in the potential state energy which, if we r.e.t.a.r.d the reaction by any suitable arrangement--such as by amalgamating the zinc and by const.i.tuting with its elements a battery which we cause to act on a resistance--may be made to exhaust itself as slowly as one may desire.
There can, therefore, be nothing in any way surprising in the fact that a combination which, like the atomic combination of radium, is not stable--since it disaggregates itself,--is capable of spontaneously liberating energy, but what may be a little astonis.h.i.+ng, at first sight, is the considerable amount of this energy.
M. Curie has calculated directly, by the aid of the calorimeter, the quant.i.ty of energy liberated, measuring it entirely in the form of heat. The disengagement of heat accounted for in a grain of radium is uniform, and amounts to 100 calories per hour. It must therefore be admitted that an atom of radium, in disaggregating itself, liberates 30,000 times more energy than a molecule of hydrogen when the latter combines with an atom of oxygen to form a molecule of water.
We may ask ourselves how the atomic edifice of the active body can be constructed, to contain so great a provision of energy. We will remark that such a question might be asked concerning cases known from the most remote antiquity, like that of the chemical systems, without any satisfactory answer ever being given. This failure surprises no one, for we get used to everything--even to defeat.
When we come to deal with a new problem we have really no right to show ourselves more exacting; yet there are found persons who refuse to admit the hypothesis of the atomic disaggregation of radium because they cannot have set before them a detailed plan of that complex whole known to us as an atom.
The most natural idea is perhaps the one suggested by comparison with those astronomical phenomena where our observation most readily allows us to comprehend the laws of motion. It corresponds likewise to the tendency ever present in the mind of man, to compare the infinitely small with the infinitely great. The atom may be regarded as a sort of solar system in which electrons in considerable numbers gravitate round the sun formed by the positive ion. It may happen that certain of these electrons are no longer retained in their orbit by the electric attraction of the rest of the atom, and may be projected from it like a small planet or comet which escapes towards the stellar s.p.a.ces. The phenomena of the emission of light compels us to think that the corpuscles revolve round the nucleus with extreme velocities, or at the rate of thousands of billions of evolutions per second. It is easy to conceive from this that, notwithstanding its lightness, an atom thus const.i.tuted may possess an enormous energy.[43]
[Footnote 43: This view of the case has been made very clear by M.
Gustave le Bon in _L'evolution de la Matiere_ (Paris, 1906). See especially pp. 36-52, where the amount of the supposed intra-atomic energy is calculated.--ED.]
Other authors imagine that the energy of the corpuscles is princ.i.p.ally due to the extremely rapid rotations of those elements on their own axes. Lord Kelvin lately drew up on another model the plan of a radioactive atom capable of ejecting an electron with a considerable _vis viva_. He supposes a spherical atom formed of concentric layers of positive and negative electricity disposed in such a way that its external action is null, and that, nevertheless, the force emanated from the centre may be repellent for certain values when the electron is within it.
The most prudent physicists and those most respectful to established principles may, without any scruples, admit the explanation of the radioactivity of radium by a dislocation of its molecular edifice. The matter of which it is const.i.tuted evolves from an admittedly unstable initial state to another stable one. It is, in a way, a slow allotropic transformation which takes place by means of a mechanism regarding which, in short, we have no more information than we have regarding other a.n.a.logous transformations. The only astonishment we can legitimately feel is derived from the thought that we are suddenly and deeply penetrating to the very heart of things.
But those persons who have a little more hardihood do not easily resist the temptation of forming daring generalisations. Thus it will occur to some that this property, already discovered in many substances where it exists in more or less striking degree, is, with differences of intensity, common to all bodies, and that we are thus confronted by a phenomenon derived from an essential quality of matter. Quite recently, Professor Rutherford has demonstrated in a fine series of experiments that the alpha particles of radium cease to ionize gases when they are made to lose their velocity, but that they do not on that account cease to exist. It may follow that many bodies emit similar particles without being easily perceived to do so; since the electric action, by which this phenomenon of radioactivity is generally manifested, would, in this case, be but very weak.
If we thus believe radioactivity to be an absolutely general phenomenon, we find ourselves face to face with a new problem. The transformation of radioactive bodies can no longer be a.s.similated to allotropic transformations, since thus no final form could ever be attained, and the disaggregation would continue indefinitely up to the complete dislocation of the atom.[44] The phenomenon might, it is true, have a duration of perhaps thousands of millions of centuries, but this duration is but a minute in the infinity of time, and matters little. Our habits of mind, if we adopt such a conception, will be none the less very deeply disturbed. We shall have to abandon the idea so instinctively dear to us that matter is the most stable thing in the universe, and to admit, on the contrary, that all bodies whatever are a kind of explosive decomposing with extreme slowness. There is in this, whatever may have been said, nothing contrary to any of the principles on which the science of energetics rests; but an hypothesis of this nature carries with it consequences which ought in the highest degree to interest the philosopher, and we all know with what alluring boldness M. Gustave Le Bon has developed all these consequences in his work on the evolution of matter.[45]
[Footnote 44: This is the main contention of M. Gustave Le Bon in his work last quoted.--ED.]
[Footnote 45: See last note.--ED.]
There is hardly a physicist who does not at the present day adopt in one shape or another the ballistic hypothesis. All new facts are co-ordinated so happily by it, that it more and more satisfies our minds; but it cannot be a.s.serted that it forces itself on our convictions with irresistible weight. Another point of view appeared more plausible and simple at the outset, when there seemed reason to consider the energy radiated by radioactive bodies as inexhaustible.
It was thought that the source of this energy was to be looked for without the atom, and this idea may perfectly well he maintained at the present day.
Radium on this hypothesis must be considered as a transformer borrowing energy from the external medium and returning it in the form of radiation. It is not impossible, even, to admit that the energy which the atom of radium withdraws from the surrounding medium may serve to keep up, not only the heat emitted and its complex radiation, but also the dissociation, supposed to be endothermic, of this atom.