Page:Catholic Encyclopedia, volume 12.djvu/93

 PHYSICS

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PHYSICS

Ruggiero Giuseppe Boscovich, S.J. (1711-87), pub- lished a detailed e.xposition of the views attacked by Buff on and defended by Clairaut, and, inspired alike by the opinions of Newton and Leibniz, he conceived a cosmology in which the universe is composed solely of material points, these being attracted to each other in pairs. When these points are separated by a sensible distance, their attraction is reduced to mere universal attraction, whereas when they are in very close proximity it assumes a dominant importance. Boscovich's cosmology prov-ided physical theory with a programme which the geometricians of the eighteenth century, and of a great portion of the nineteenth, laboured assiduously to carry out.

The efforts of Johann Andreas von Segner (1704- 77), and subsequently of Thomas Young (1773-1829), again drew attention to capillary phenomena, and with the assistance of the hj'pothesis of molecular attraction, as also of Clairaut's method, Laj)lace advanced in 1806 and 1807 an admirable theory, which Karl Friedrich Gauss (1777-1855) improved in 1829. Being a thoroughly-convinced partisan of Boscovich's cosmological doctrine, Laplace com- municated his convictions to numerous geometricians, who surrendered to the ascendency of his genius; we shall only mention Claude-Louis-Marie Navier (1785- 1836), Poisson, and Augustin Cauchy (1789-1857). In developing the consequences of the hypothesis of molecular attraction Navier, Poisson, and Cauchy succeeded in buiUling up the theory of the equilibrium and small motions of elastic bodies, one of the finest and most fruitful theories of modern physics. The discredit into which the progress of present-day thermodynamics has brought Boscovich's cosmology has, however, affected scarcely anything of what Laplace, Gauss, Navier, Poisson, Cauchy, and many others have deduced from the principles of this cosmology. The theories which they established have always been readily justified with the assistance of new methods, the way of bringing about this justi- fication having been indicated by Cauchy himself and George Green. After Macquer, many chemists used the hj'pothesis of molecular attraction in an attempt to disentangle the laws of reaction which they studied, and among these scientists we may men- tion Torbern Bergman (1735-1784), and above all Claude-Louis BerthoUet (1784-1822). When the latter published his "Statique chimique" in 1803, he believed that the science of chemical equilibria, sub- ject at last to Newton's method, had found its true direction; however, it was not to enter upon this direction until much later on, when it would be guided by precepts altogether different and which were to be formulated by thermodynamics.

XXVIIL Revival of the Undulatort Theoet OF Light. — The emission theory of Ught not only led Newton to conceive the hj-pothesis of molecular attraction, but seemed to provide this hypothesis with an opportunity for further success by permitting Laplace to find, in the emission system, the laws of the double refraction of Iceland spar, which laws Huygens had discovered by the use of the undulatory theorj'. In this way Newton's optics appeared to rob Huygens's optics of the one ad\'antage in which it glorified. However, at the very moment that La- place's discovery seemed to ensure the triumph of the emission system, the undulatory theory carried off new and dazzling victories, won mainly through the efforts of Thomas Young and Augustin-Jean Fresnel (1788-1827). Between 1801 and 1803 Young made the memorable discoveries which provoked this revi- val of undulatory optics. The comparison of the ether that vibrates in a ray of light to the air that vibrates in a resonant tube led him to ex-plain the alternately light and dark fringes that show in a place illumined by two etiual bcam.s slightly inclined to each other. The principle of interference, thus justified, allowed XII.— 5.

him to connect with the undulatory theory the expla- nation of the colours of thin laminse that Newton had demanded of the "fits of easy transmission and easy reflection" of the particles of light.

In 1815 Fresnel, who combined this principle of interference with the methods devised by Huygens, took up the theory of the phenomena of diffraction which had been discovered by Francesco Maria Gri- maldi, S.J. (1618-63), and had remained a mystery to opticians. Fresnel's attempts at explaining these phenomena led him to draw up in 1818 a memoir which in a marked degree revealed the essential char- acter of his genius, namely, a strange power of divina- tion exercised independently of all rules of deductive reasoning. Despite the irregularity of his procedure, Fresnel made known very complicated formulae, the most minute details of which were verified by experi- ment, and long afterwards justified according to the logical method of mathematicians. Never did physi- cist conquer more important and more unthought-of truths, and yet never was there emjiloyed a method more capable of leading the common mind into error. Up to this time the vibrations of ether in a ray of light had been supposed to be longitudinal, as it is in the air of a resonant tube, but in 1808 Etienne-Louis Malus (1775-1812) discovered the polarization of Ught when reflected on glass, and, in 1817, when studying this phenomenon, Young was led to suppose that luminous vibrations are perpendicular to the ray which transmits them. Fresnel, who had con- ceived the same idea, completed an experiment (1816) in collaboration with Arago (1786-1853), which proved the view that luminous vibrations are trans- verse to the direction of propagation.

The hypothesis of transverse vibrations was, for Fresnel, the key to all the secrets of optics, and from the day that he adopted it he made discoveries with great rapidity. Among these discoveries were: (a) The complete theory of the phenomena of polarization accompanying the reflection or refraction of light on the surface of contact of two isotropic media. The peculiarities which accompany total reflection gave Fresnel an opportunity to display in a most striking manner his strange power of divination and thus throw out a veritable challenge to logic. This divi- nation was no less efficient in the second discovery, (b). In studying double refraction, Huygens limited himself to determining the direction of luminous rays in the interior of crystals now called uniaxial, without, however, being able to account for the polarization of these rays; but with the aid of the wave-surface, Fresnel succeeded in giving the most elegant form to the law of the refraction of rays in biaxial crystals, and in formulating rules by which rays polarize in the interior of all crystals, uniaxial as well as biaxial.

Although all these wonderful theories destroyed the theory of emission, the hypothesis of molecular attraction was far from losing ground. In fact Fresnel thought he could find in the elasticity of the ether, which transmits luminous vibrations, the explanation of all the optical laws that he had verified by experi- ment, and he .sought the explanation of this elasticity and its laws in the attraction which he believed to exist between the contiguous particles of this fluid. Being too little of a mathematician and too Uttle of a mechanician to go very far in the analysis of such a problem, he left its solution to his successors. To this task, so clearly defined by Fresnel, Cauchy de- voted the most powerful efforts of his genius as an algebraist and, thanks to this pupil of Laplace, the Newtonian physics of molecular attraction became an active factor in the propagation of the theory of undulatory optics. Fresnel's discoveries did not please all Newtonians as much as they did Cauchy. Arago could never admit that luminous vibrations were transverse, notwithstanding that he had collab- orated with Fresnel in making the experiment by