690 X. ON THE STRUCTURE OF 'MATTER'
electrons. These and other structural considerations led Rutherford to propose a different atomic model, which was much more successful for a while.
The Rutherford atom is supposed to be composed of a nucleus of positive electricity surrounded by negative electrons. The simplest atom is that of hydrogen, and was assumed to consist of one electron revolving around the simplest positive nucleus or proton, each having a charge, e.s.u.,
and different masses, the mass of the nucleus or proton being 1845 times the mass of the electron. The other atoms represent more complex structures built up of protons and electrons, into the details of which we need not enter here. But this theory encountered difficulties of theoretical as well as of experimental character. Niels Bohr eliminated most of them by the application of the quantum theory to the atom.
For simplicity of writing in that which follows, I will use a descriptive language omitting in each statement 'we assume' . , but the reader should be continuously aware, that when we deal with the sub-microscopic levels we deal only with inferential units the representation of which involves a great many assumptions. For my purpose it is enough to stress: (1) the negative fact, that the structure of materials is definitely different from that which was assumed before the advent of the quantum theories, (2) that in science, inferential units represent abstractions of higher order and are as reliable as the lower order abstractions which we gather on the macroscopic levels, if they are treated semantically as hypothetical units. The layman should realize that his 'world-outlook' appears as full of assumptions as any scientific one, except that his assumptions are not conscious and cannot be verified, whereas most of the scientific assumptions are conscious and are continually verified.
In the older theory the orbits of the electrons were supposed to be arbitrary; in the Bohr theory the orbits have precedence, for which a definite magnitude, a whole number multiple of the elementary quantum of action is specified. We posit one-quantum orbits, two-quantum orbits . , to which definite values of the orbits, the velocity, the number of revolutions, and the energy correspond. In a one-quantum orbit, for instance, the velocity is supposed to be equal to
c/140, that is one 140th of the velocity of light, and the number of revolutions equal to 6000 billions a second.
Bohr later modified his atomic model structurally by taking into account the movement of the nucleus. The electron was not supposed to revolve any longer around the proton, but both proton and electron were assumed to revolve around their common centre of gravity. In the simplest form the Bohr atomic model is shown in Fig. 3, representing the atom of hydrogen, which we assume to consist of a nucleus with one positive