or atoms in space which are as regular and as important as those assigned by the astronomers to the movements and arrangements of the heavenly bodies. If bodies are formed out of elements that combine with one another on some fixed and definite principle, and not under pressure of the law of gravitation, if this combination is universal, then there must be some law more elemental than that involved in gravity to explain this combination, to account for the phenomena of magnetism and electricity, to explain the disclosures of the microscope in that molecular world which underlies all that the most delicate instruments can render visible. For these and similar reasons the atomic theory of the universe has been proposed. Yet it is recognized as exceedingly complicated and not without need of modification.
But while this theory is held by large numbers of eminent scientists, since 1860 the study of gases has brought about a modification of it and given birth to the kinetic theory of the universe—the old theory of Heraclitus of Ephesus adapted to modern conditions—viz., that all things are in motion. The experiments of Clausius of Zürich, of James Clerk Maxwell of Scotland, and the practical applications of their theories and his own by Joule of Manchester in the study of heat, made it clear that what seems to be the dead pressure of the gases is an apparent rest of particles which are in reality engaged in a constant and law-determined bombardment of each other.
These particles, it was shown, move laterally and with well-nigh incredible speed. It is motion which reveals this long-hidden secret of the gases. It is motion also which accounts for the rigidity of solid bodies. The theory of motion, based on the doctrine of gravity as set forth by Newton and his successors, was the foundation of astronomy. That theory received the support of Huyghens of Holland, of Euler of Berlin and St. Petersburg, the famous mathematician, and was accepted by Young, Count Bumford and Fresnel. Young's undulatory theory of light was made known to the world during the decade from 1791 to 1801. The kinetic theory, the theory of motion everywhere, motion directed, determined and controlled by fixed law, received hesitatingly at first, was steadily opposed by Laplace, but was at last made popular by Arago and Fresnel in France. The doctrine of the polarization of light proved, it is affirmed, by the interference of light waves, contributed to its acceptance. Still opinions even in the Paris Academy were unsettled and confused. Hence the offer by the academy of a prize for an essay which should consider this whole subject, weigh calmly conflicting theories, discover the truth or the falsehood in each one of them, and with all possible thoroughness subject this new theory of motion to the severest tests. At the request of the academy, but against his wishes, Fresnel undertook the investigation. He began his studies with the conviction that this new theory had little foundation upon which to rest. But after careful experiment
