MODERN THOUGHT 139 



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 uni- 

 verse has been proposed. Yet it is recognized as exceedingly compli- 

 cated and not without need of modification. 



But while this theory is held by large numbers of eminent scien- 

 tists, 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 Zurich, 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 appar- 

 ent 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 ac- 

 cepted 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 sub- 

 ject, weigh calmly conflicting theories, discover the truth or the 

 falsehood in each one of them, and with all possible thoroughness sub- 

 ject this new theory of motion to the severest tests. At the request of 

 the academy, but against his wishes, Fresnel undertook the investiga- 

 tion. He began his studies with the conviction that this new theory 

 had little foundation upon which to rest. But after careful experiment 



