EXCLUSION PRINCIPLE AND QUANTUM MECHANICS 35 parallel proton spins; para-H2; even rotational quantum numbers, antipar allel spins)was not yet reached. As you know, this hypothesis was later confirmed by the experiments of Bonhoeffer and Harteck and of Eucken, which showed the theoretically predicted slow transformation of one mod ification into the other Among the symmetry classes for other nuclei those with a different parity of their mass number M and their charge number Z are of a particular in- terest. If we consider a compound system consisting of numbers A, A2 of different constituents, each of which is fulfilling the exclusion principle, and a number S of constituents with symmetrical states, one has to expect symmetrical or antisymmetrical states if the sum A, A,+... is even odd. This holds regardless of the parity of S. Earlier one tried the assumption of protons and electrons, so that M is the number of pro- tons,M-Z the number of electrons in the nucleus. It had to be expected then that the parity of z determines the symmetry class of the whole nucleus Already for some time the counter-example of nitrogen has been known to ave the spin iand symmetrical states. After the discovery of the neutron, the nuclei have been considered, however, as composed of protons and neu trons in such a way that a nucleus with mass number M and charge number Z should consist of Z protons and M-Z neutrons. In case the neutrons would have symmetrical states, one should again expect that the parity of the charge number Z determines the symmetry class of the nuclei. If, how ever, the neutrons fulfill the exclusion principle, it has to be expected that the parity of M determines the symmetry class For an even M, one should always have symmetrical states, for an odd M, antisymmetrical ones. It was the latter rule that was confirmed by experiment without exception, thus proving that the neutrons fulfill the exclusion principle The most important and most simple crucial example for a nucleus with a different parity of M and Z is the heavy hydrogen or deuteron with M=2 and Z= l which has symmetrical states and the spin 1=1, as could be proved by the investigation of the band spectra of a molecule with two deu- tenons. From the spin value iof the deuteron can be concluded that the neutron must have a half-integer spin. The simplest possible assumption that nis spin of the neutron is equal to 12, just as the spin of the proton and of th electron turned out to be correct. There is hope, that further experiments with light nuclei, especially with protons, neutrons, and deuterons will give us further information about the nature of the forces between the constituents of the nuclei, which, at present,EXCLUSION PRINCIPLE AND QUANTUM MECHANIC S 35 parallel proton spins; para-H2: even rotational quantum numbers, antipar￾allel spins) was not yet reached. As you know, this hypothesis was later, confirmed by the experiments of Bonhoeffer and Harteck and of Eucken, which showed the theoretically predicted slow transformation of one mod￾ification into the other. Among the symmetry classes for other nuclei those with a different parity of their mass number M and their charge number Z are of a particular in￾terest. If we consider a compound system consisting of numbers A1, A2, . . . of different constituents, each of which is fulfilling the exclusion principle, and a number S of constituents with symmetrical states, one has to expect symmetrical or antisymmetrical states if the sum AI + A2 + . . . is even or odd. This holds regardless of the parity of S. Earlier one tried the assumption that nuclei consist of protons and electrons, so that M is the number of pro￾tons, M - Z the number of electrons in the nucleus. It had to be expected then that the parity of Z determines the symmetry class of the whole nucleus. Already for some time the counter-example of nitrogen has been known to have the spin I and symmetrical states 18. After the discovery of the neutron, the nuclei have been considered, however, as composed of protons and neu￾trons in such a way that a nucleus with mass number M and charge number Z should consist of Z protons and M - Z neutrons. In case the neutrons would have symmetrical states, one should again expect that the parity of the charge number Z determines the symmetry class of the nuclei. If, how￾ever, the neutrons fulfill the exclusion principle, it has to be expected that the parity of M determines the symmetry class : For an even M, one should always have symmetrical states, for an odd M, antisymmetrical ones. It was the latter rule that was confirmed by experiment without exception, thus proving that the neutrons fulfill the exclusion principle. The most important and most simple crucial example for a nucleus with a different parity of M and Z is the heavy hydrogen or deuteron with M = 2 and Z = 1 which has symmetrical states and the spin I = 1, as could be proved by the investigation of the band spectra of a molecule with two deu￾terons 19. From the spin value I of the deuteron can be concluded that the neutron must have a half-integer spin. The simplest possible assumption that this spin of the neutron is equal to ½, just as the spin of the proton and of the electron, turned out to be correct. There is hope, that further experiments with light nuclei, especially with protons, neutrons, and deuterons will give us further information about the nature of the forces between the constituents of the nuclei, which, at present