157Chapter 17The Structure of the Atom
Early in the 1800's, when John Dalton proposed the atomic structure of
matter; he considered the atom to be the simplest structure possible in
nature, the ultimate subdivision of matter. The indivisibility and
immutability of these ultimate building blocks of nature were implied in
the very term, atom. The atom, as originally defined by the early Greeks,
Leucippus and Democritus, were also thought to be indivisible and
unchanging. Dalton had no notion that the atom was composed of the
still smaller particles, the electron, the proton and the neutron. The
phenomenon of electricity was not connected to the existence of
electrons and protons or to any charged particle, for that matter. Rather,
it was believed that electrical phenomena could be explained in terms of
two different kinds of electric fluids, one positive and the other negative.
The term current itself, used to describe a flow of electrically charged
particles, retains the original connotation of the fluid nature of electricity.
The electric fluid used to describe charge was considered to be massless
and was not associated with matter or atoms. It was an independent
constituent of nature.
Our description of electric and magnetic phenomena in Chapter 10
was not really historically accurate. The laws of electricity and
magnetism were originally formulated in terms of the electric fluids
rather than in terms of charged particles. All of the notions of
electromagnetic phenomena, including Maxwell’s equations, were
conceived in terms of the density and the current of the positive and
negative electric fluids. It was not until after the electromagnetic
interaction was understood that it was realized that the density of the two
electric fluids was the density of electrons and protons and that currents
were generated by the movement of these particles. Just as the notion of
caloric fluid disappeared once it was realized that heat was due to the