For under certain conditions the chemical atoms emit light waves of a specific length or oscillation frequency - their familiar characteristic spectra - and these can come in the form of electromagnetic waves only from accelerated electric quanta.

The most common and most important result of them is that the nature and size of the effect on corresponding series of different elements are largely an expression of the peculiarity of their atomic structure - or, at least, of the structure of the surface.

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.

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.

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.

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

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.