TABLE OF CONTENTS One of the first important clues to understanding atomic structure involved the study of the emission and absorption of light by the atoms of different elements. Physicists knew from Maxwell's theory that light is emitted and absorbed only by accelerating charges. This suggested that the atom might contain moving charges. Patterns and regularities in the properties of the light emitted we expected to provide valuable clues about the precise nature of the motions of the moving charges. The results of this study were so important to the unraveling of atomic structure that we review their development here in some detail.
It has long been known that light is emitted by gases or vapors when they are excited in any one of several ways:
by heating the gas to a high temperature, as when a volatile substance is put into a flame;
by an electric discharge through gas in the space between the terminals of an electric arc; or
by a continuous electric current in a gas at low pressure, as in the now familiar "neon sign."
The Scottish physicist Thomas Melvill made the pioneering experiments on light emitted by various excited gases in 1752. He put one substance after another in a flame, "having placed a pasteboard with a circular hole in it between my eye and the flame , I examined the constitution of these different lights with a prism." Melvill found that the spectrum of visible light from a hot gas...
