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8 ON THE SPECTBTJM OF HYDEOGEN

atom is exactly equal to this number. This hypothesis, which was
first stated by van den Broek (1912), opens the possibility of
obtaining a simple explanation of the periodic system. This as-
sumption is strongly confirmed by experiments on the elements
of small atomic weight. In the first place, it is evident that ac-
cording to Rutherford's theory the a particle is the same as the
nucleus of a helium atom. -Since the a particle has a double positive
charge it follows immediately that a neutral helium atom contains
two electrons. Further the concordant resul-ts obtained from cal-
culations based on experiments as different as the diffuse scatter-
ing of X-rays and the decrease in velocity of a rays in passing
through matter render the conclusion extremely likely that a
hydrogen atom contains only a single electron. This agrees most
beautifully with the fact that 3. J. Thomson in his well-known
experiments on rays of positive electricity has never observed a
hydrogen atom with more than a single positive charge, while all
other elements investigated may have several charges.

Let us now assume that a hydrogen atom simply consists of an
electron revolving around a nucleus of equal and opposite charge,
and of a mass which is very large in comparison with that of the
electron. It is evident that this assumption may explain the peculiar
position already referred to which hydrogen occupies among the
elements, but it appears at the outset completely hopeless to attempt
to explain anything at all of the special properties of hydrogen,
still less its line spectrum, on the basis of considerations relating
to such a simple system.

Let us assume for the sake of brevity that the mass of the nucleus
is infinitely large in proportion to that of the electron, and that the
velocity of the electron is very small in comparison with that of
light. If we now temporarily disregard the energy radiation, which,
according to the ordinary electrodynamics, will accompany the ac-
celerated motion of the electron, the latter in accordance with
Kepler's first law will describe an ellipse with the nucleus in one
of the foci. Denoting the .frequency of revolution by o>, and the
major axis of the ellipse by 2a we find that



	8 ON THE SPECTBTJM OF HYDEOGEN atom is exactly equal to this number. This hypothesis, which was first stated by van den Broek (1912), opens the possibility of obtaining a simple explanation of the periodic system. This as- sumption is strongly confirmed by experiments on the elements of small atomic weight. In the first place, it is evident that ac- cording to Rutherford's theory the a particle is the same as the nucleus of a helium atom. -Since the a particle has a double positive charge it follows immediately that a neutral helium atom contains two electrons. Further the concordant resul-ts obtained from cal- culations based on experiments as different as the diffuse scatter- ing of X-rays and the decrease in velocity of a rays in passing through matter render the conclusion extremely likely that a hydrogen atom contains only a single electron. This agrees most beautifully with the fact that 3. J. Thomson in his well-known experiments on rays of positive electricity has never observed a hydrogen atom with more than a single positive charge, while all other elements investigated may have several charges. Let us now assume that a hydrogen atom simply consists of an electron revolving around a nucleus of equal and opposite charge, and of a mass which is very large in comparison with that of the electron. It is evident that this assumption may explain the peculiar position already referred to which hydrogen occupies among the elements, but it appears at the outset completely hopeless to attempt to explain anything at all of the special properties of hydrogen, still less its line spectrum, on the basis of considerations relating to such a simple system. Let us assume for the sake of brevity that the mass of the nucleus is infinitely large in proportion to that of the electron, and that the velocity of the electron is very small in comparison with that of light. If we now temporarily disregard the energy radiation, which, according to the ordinary electrodynamics, will accompany the ac- celerated motion of the electron, the latter in accordance with Kepler's first law will describe an ellipse with the nucleus in one of the foci. Denoting the .frequency of revolution by o>, and the major axis of the ellipse by 2a we find that