its counterpart, the positive electron of very small mass, exists. The unit of positive electricity has never been found to be as- sociated with a mass less than that of the hydrogen atom. This has led to the view that the hydrogen nucleus is the positive electron, and that its mass is about 1,845 times that of the nega- tive electron. This difference in mass between the units of positive and negative electricity appears to be fundamental, and offers an explanation of the asymmetrical distribution of positive and negative electricity in the structure of atoms.
Since the helium nucleus has a mass 4 and charge 2, it should be composed of four hydrogen nuclei and two electrons. Its mass, however, is less than that of four free hydrogen nuclei. Such a change of mass in the very close combinations of positive and negative nuclei is to be expected. According to the theory of relativity energy has mass, and the loss of mass TO of a system is numerically given by E=mc z where E is the energy liberated and c the velocity of light. On this view the combination of the positive and negative electrons to form the helium nucleus is accompanied by a large release of energy. From the difference between the mass of the helium nucleus and that of four hydro- gen nuclei, it can readily be calculated that the helium nucleus is such a stable combination that an amount of energy corre- sponding to four or five a particles from radium would be required to dissociate it. The difference between the masses of the pro- tons in the nucleus and free hydrogen nuclei is thus to be ascribed in general to the close packing of the positive and negative units composing the nucleus.
On the views outlined above the number of electrons in any nucleus can at once be calculated. For example, oxygen of nuclear charge 8 should be made up of 16 positive units and 8 electrons. For such a nucleus to hold together it seems clear that the forces between the charged units at such small distances must be different from that of the inverse square. While it has been experimentally shown that the law of the inverse square holds at any rate approximately close to the nucleus of a heavy atom like gold, this law breaks down in very close collisions of light atoms where the nuclei approach very close to each other. For example, it has been found that the number of hydrogen atoms which are set in swift motion when a particles pass through hydrogen is very different from that to be expected if the nuclei behave as point charges repelling each other according to the law of the inverse square. The experimental information at present available is too indefinite to hazard more than a guess as to the nature and magnitude of the forces that come into play when nuclei approach very close to one another, as they must do in the structure of the nucleus of a heavy atom.
Stability of Atoms. Apart from the heavy radioactive elements which belong to a class by themselves, and two other elements potassium and rubidium which spontaneously emit swift electrons, the atoms of the ordinary, elements appear to be very stable structures which cannot be broken up by ordinary chem- ical and physical agencies. Some experiments have suggested that possibly helium and hydrogen may be liberated by the passage of an electric discharge through gases, but on account of the presence of these elements in many materials it is difficult to prove definitely that they arise from artificial transformation. In considering the possibility of the disintegration of elements it should be borne in mind that the loss of one or more electrons from the outer electronic system has no permanent effect on the atom, for other electrons ultimately fall into the atom to fill their place. In order to produce a permanent transformation of the atom it appears necessary to remove a positively charged particle or an electron from the nucleus of the atom. This can only be effected by agencies which are able to penetrate the nucleus or to pass very close to its structure.
The a particle expelled from radium is one of the most con- centrated sources of energy known to us, and on account of its speed should be able to penetrate the structure of the nuclei of many of the lighter atoms and still retain sufficient energy to disrupt the bonds that hold the parts of the nucleus together. In the case of an atom of high nuclear charge the a particle may lose so much of its energy in approaching the nucleus that it may
be unable to effect its disintegration. It has been found that when o particles pass through hydrogen or any material con- taining combined hydrogen some of the particles pass so close to the hydrogen nucleus that they set it in swift motion. These swift hydrogen atoms can be detected by the scintillations they produce on a zinc-sulphide screen. This is purely a case of collisions of atomic nuclei, and the speed of the " H " atom set in motion can be calculated by the ordinary laws of mechanics. The maximum range or distance of penetration of such a particle is about four times that of the incident a particle.
In a similar way other nuclei must be set in swift motion by their collision with a particles, but it can be calculated that in most cases such nuclei are unable to travel as far as the a particle, and thus remain undetected amid the great number of incident a particles.
When a strong beam of a rays passes through oxygen or carbon dioxide only a few H atoms are observed, and these appear to come from the radioactive source. When, however, the rays pass through dry nitrogen a much greater number of penetrating particles is observed. Rutherford has shown by the action of a magnetic field that these particles are not atoms of nitrogen but probably charged atoms of hydrogen. Rutherford and Chadwick have tested a number of elements in this way and have found that, in addition to nitrogen, boron, fluorine, sodium and phos- phorus show a similar property. As far as observation has gone it seems probable that these expelled particles are H atoms which are released by the disintegration of the nucleus. The velocity of expulsion of such H atoms is greater than that of an H atom in a direct collision with an a particle. For example, using a particles of range 7-0 cm. in air, ordinary H atoms travel 29 cm. in air while the atoms from nitrogen go 40 cm., and those from aluminium not less than 80 cm. It thus seems clear that the effects observed in nitrogen and aluminium cannot be ascribed to ordinary hydrogen as an impurity. It is of interest to note that if the particle from aluminium is an if atom it is released with more energy than that of the incident a particle. Elements like carbon, oxygen, and sulphur, whose atomic mass is given by 4 where n is a whole number, do not give rise to H atoms, but only those elements whose mass is giv^n by 4^+2 or 471+3. It thus seems clear that a disintegration of certain atoms can be produced by the intense collisions with the a particle in which an H atom is released with great velocity. General evidence indicates that not only H atoms but possibly also atoms of mass 3 or 4 may be liberated in a similar way, but the experimental evidence was in 1921 too indefinite for any certain conclusion.
It should be borne in mind that the disintegration observed in this way is on an exceedingly small scale. Not more than one . particle in a million gets sufficiently close to a nucleus to release an H atom. It seems clear, however, that while the ordinary atom is undoubtedly very stable, its disintegration can be brought about by the aid of sufficiently powerful agencies which are able to penetrate its structure. As already pointed out, there are strong reasons for believing that the helium nucleus is a very stable structure which cannot be broken up even by the swift- est a particle at our disposal.
While it is reasonable to suppose that all the elements have been built up by combinations of protons and electrons, there was in 1921 no experimental evidence to throw light on the conditions necessary to lead to the formation of complex nuclei. No doubt, however, this process of aggregation has gone on in the past, and may still be in progress under favourable conditions, if not on this earth at any rate on some of the stars. (E. Ru.)
MAUBEUGE, SIEGE OF (1914). The fortress of Maubeuge, which in oldtime wars played an important role as commanding the routes leading from the Spanish or Austrian Netherlands to the Oise valley, was reconstructed as a ring fortress of the modern type between 1878 and 1896, under the defence scheme of Gen. Sere de Rivieres. Its function, like that of Besancon on the other flank of the French eastern front, was in substance to absorb the forces of a German army which should seek to turn the flank of the Lorraine-Meuse defence. The great development of communications of all sorts in north-eastern France
