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Forms Of Matter

The development of physics both contributed to and depended on ideas about the structure of matter. In this regard, the history of physics is tied to the history of chemistry. Both sciences inherited a debate that began with the ancients regarding atomism versus continuity. Combining the influences of, among others, Pythagoras and Democritus, Plato saw matter as being composed of atoms that had different geometrical shapes for each of the four elements. Against this, Aristotle developed a continuum theory of matter, in part because his theory of motion would be contradicted by the existence of a void. This debate was reawakened during the sixteenth and seventeenth centuries. On the one hand, Descartes embraced a continuum theory involving a plenum of fine matter and vortices, founded on the idea that motion is caused through contact. On the other hand, Robert Boyle proposed atomistic explanations of his finding that reducing the volume of a gas increased its pressure proportionately. Newton refined Boyle's ideas by interpreting pressure as being due to mutually repelling atoms, and recommended an atomistic stance for further research in chemistry and optics.

During the eighteenth and nineteenth centuries, many theorists and experimentalists posited the existence of a number of "imponderables," substances that could produce physical effects but could not be weighed. The first of these was proposed in 1703 by the German physician and chemist Georg Ernst Stahl in order to explain the processes of oxidation and respiration. Stahl's phlogiston theory, and the renewed interest in Newton's theories of an ether medium for gravity, encouraged further theories involving imponderables, most notably electrical fire (to describe the flow of static electricity) and caloric (to describe heat flow). Although the imponderables were eventually rejected, they served as useful heuristic devices in quantifying physical laws. For example, the Scottish chemist Joseph Black (1728–1799) used the caloric theory to found the study of calorimetry and to measure specific heat (the heat required to raise the temperature of a substance one degree), and latent heat (the heat required for a substance to change its state).

Even after the work of John Dalton, few chemists and physicists before 1890 accepted the actual existence of atoms. Nevertheless, they found the atomic hypothesis to be useful in suggesting experiments and interpreting the results. In 1863, the Irish physical chemist Thomas Andrews experimentally characterized the "critical point" between the gas and liquid phases: at relatively low temperatures, as the pressure was increased, the change from gas to liquid was abrupt; however, at relatively high temperatures, the transition was continuous. In part to account for the behavior of the critical point, the Dutch physicist Johannes Diderik van der Waals (1837–1923) assumed that the forces between atoms were attractive at large range but repulsive at short range. The work of van der Waals represented the first successful theory of phase transitions and showed how an atomistic model could describe both phases.

In the mid-nineteenth century, Michael Faraday and Julius Plücker (1801–1868), among others, pioneered research on the discharge of electricity through partially evacuated glass tubes. The British chemist William Crookes made a number of improvements to these discharge tubes and called the glowing material that formed in them the "fourth state of matter" (which was later dubbed "plasma" by the American chemist Irving Langmuir). Work in this area eventually led to Joseph John Thomson's discovery of the electron and Philipp Lenard's characterization, in 1899, of the photoelectric effect.

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Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck lengthPhysics - Middle Ages, Sixteenth And Seventeenth Centuries, Eighteenth Century, Nineteenth Century, Causes Of Motion: Medieval Understandings