The question of the composition of perceptible objects is one which already occupied the mind of the ancient Greeks.

We have learnt through experience that when an electrical ray strikes the surface of an atom, an electron, and in some circumstances a second and even a third electron, can be detached.

The emitters of the spectral series are without exception single atoms, not compounds of atoms.

The removal of an electron from the surface of an atom - that is, the ionization of the atom - means a fundamental structural change in its surface layer.

It is more likely that more than a century will pass before we know the structure of the chemical atoms as thoroughly as we do our solar system.

An external electric field, meeting it and passing through it, affects the negative as much as the positive quanta of the atom, and pushes the former to one side, and the latter in the other direction.

Moreover, the abundance of chemical compounds and their importance in daily life hindered the chemist from investigating the question, in what does the individuality of the atoms of different elements consist.