of its approach to the alpha particle, it can be simply calculated that the centers of the nuclei must have passed within cm. of each other. This is an extraordinarily small distance, even smaller than the diameter of the electron itself. It is thus clear that the nuclei of hydrogen and of helium must be exceedingly minute. It should be borne in mind that such observations only give a maximum limit to the size of the nucleus, and there is no experimental evidence against the view that the nucleus of the hydrogen atom may not actually prove to be minute in volume compared even with the negative electron. If this be the case, it appears probable that the hydrogen nucleus is the positive electron and that its great mass compared with the negative electron is due to the greater concentration of its charge. According to modern theory, the electrical mass of a charged particle varies inversely as its radius. The greater mass of the positive than of the negative electron would thus be explained if its radius were only 1/1800 of that of the negative electron, viz., about cm.
There is no evidence to contradict this point of view, and its simplicity has much to commend it. In viewing the essential differences exhibited by positive and negative electricity in connection with matter and the obvious asymmetry of the distribution of the two electricities in the atom, one is driven to the conclusion that there is a fundamental distinction between positive and negative electricity. Since the unit of positive charge is identical in magnitude with the unit of negative charge, the only possible difference is the mass of the two units, and this on modern views is mainly dependent on the dimensions or degree of concentration of the electricity in these fundamental entities.
If we take the view that the hydrogen nucleus is the positive electron, it is to be anticipated that the nuclei of all atoms are built up of positive and negative electrons, the positive electricity being always in excess, so that the nucleus shows a resultant positive charge. The mass of the atom will depend mainly on the number of the massive positive electrons in the nucleus, although it will be affected to a slight extent by the number of the lighter negative electrons involved in the structure of the whole atom. The mass of the atom will no doubt be influenced also by the distribution of the positive and negative electrons in the nucleus, for these must be packed so closely together that their field must interact. As Lorentz has shown, the mass of a number of closely packed electrons is not necessarily the same as the sum of individual masses of the component electrons. Taking such factors into account, we should not necessarily expect the mass of all atoms to be nearly an integral multiple of the mass of the hydrogen atom, although it is known that in a number of cases such a relation appears to hold fairly closely.
The appearance of a helium atom in such a fundamental process as the transformation of radioactive atoms indicates that helium is one
