<P> While the 19th century had seen the success of the wave theory at describing light, it had also witnessed the rise of the atomic theory at describing matter . Antoine Lavoisier deduced the law of conservation of mass and categorized many new chemical elements and compounds; and Joseph Louis Proust advanced chemistry towards the atom by showing that elements combined in definite proportions . This led John Dalton to propose that elements were invisible sub components; Amedeo Avogadro discovered diatomic gases and completed the basic atomic theory, allowing the correct molecular formulae of most known compounds--as well as the correct weights of atoms--to be deduced and categorized in a consistent manner . Dimitri Mendeleev saw an order in recurring chemical properties, and created a table presenting the elements in unprecedented order and symmetry . </P> <P> At the close of the 19th century, the reductionism of atomic theory began to advance into the atom itself; determining, through physics, the nature of the atom and the operation of chemical reactions . Electricity, first thought to be a fluid, was now understood to consist of particles called electrons . This was first demonstrated by J.J. Thomson in 1897 when, using a cathode ray tube, he found that an electrical charge would travel across a vacuum (which would possess infinite resistance in classical theory). Since the vacuum offered no medium for an electric fluid to travel, this discovery could only be explained via a particle carrying a negative charge and moving through the vacuum . This electron flew in the face of classical electrodynamics, which had successfully treated electricity as a fluid for many years (leading to the invention of batteries, electric motors, dynamos, and arc lamps). More importantly, the intimate relation between electric charge and electromagnetism had been well documented following the discoveries of Michael Faraday and James Clerk Maxwell . Since electromagnetism was known to be a wave generated by a changing electric or magnetic field (a continuous, wave - like entity itself) an atomic / particle description of electricity and charge was a non sequitur . Furthermore, classical electrodynamics was not the only classical theory rendered incomplete . </P> <P> In 1901, Max Planck published an analysis that succeeded in reproducing the observed spectrum of light emitted by a glowing object . To accomplish this, Planck had to make an ad hoc mathematical assumption of quantized energy of the oscillators (atoms of the black body) that emit radiation . Einstein later proposed that electromagnetic radiation itself is quantized, not the energy of radiating atoms . </P> <P> Black - body radiation, the emission of electromagnetic energy due to an object's heat, could not be explained from classical arguments alone . The equipartition theorem of classical mechanics, the basis of all classical thermodynamic theories, stated that an object's energy is partitioned equally among the object's vibrational modes . But applying the same reasoning to the electromagnetic emission of such a thermal object was not so successful . That thermal objects emit light had been long known . Since light was known to be waves of electromagnetism, physicists hoped to describe this emission via classical laws . This became known as the black body problem . Since the equipartition theorem worked so well in describing the vibrational modes of the thermal object itself, it was natural to assume that it would perform equally well in describing the radiative emission of such objects . But a problem quickly arose: if each mode received an equal partition of energy, the short wavelength modes would consume all the energy . This became clear when plotting the Rayleigh--Jeans law which, while correctly predicting the intensity of long wavelength emissions, predicted infinite total energy as the intensity diverges to infinity for short wavelengths . This became known as the ultraviolet catastrophe . </P>

Who discovered that light was a particle and how