Spinning atoms and spinning electrons

NO DOUBT you are all accustomed to thinking of atoms as objects — very small objects, of course — which are endowed with weight. I can say that with perfect safety to an audience of engineers and physicists; but indeed it can be said with safety to any audience — I mean, of course, any audience literate enough to attach any meaning at all to such a word as “atom.” It may be that philosophers of the past have imagined weightless atoms — I am not historian enough to deny that, nor to affirm it; but if such have ever been invented, they have remained quite outside the currents of modern thought. For us, weight is a property which we attribute to the atom. Since this is, after all, a professional audience, I will now change over to that other word which many people have such difficulty in distinguishing from “weight”: I will say that mass is a property which we attribute to the atom. In a way, that is a negative statement. It means that we do not hope to explain mass in terms of something more fundamental; it means that we accept mass as being itself so fundamental that even the elementary particles have it. When I say “elementary particles,” I am still referring in part to the atoms, though it is a somewhat careless usage to do so; but I am referring also to electrons both positive and negative, to protons, to alpha-particles, to nuclei — to all the particles, in effect, of which the atoms are built up. Also I ought to include the corpuscles of light, but this lecture will be quite long enough if I leave them almost unmentioned. All of these particles, then, are endowed with mass; each of them has a characteristic mass of its own, which we do not attempt to explain, but which we do try to measure as closely as we can.