History of the Intellectual Development of Europe, Volume II (of 2) / Revised Edition

by John William Draper

Theories of electricity.It is an illustration of the excellence and fertility of modern methods that the phenomena of the attraction displayed by amber, which had been known and neglected for two thousand years, in one-tenth of that time led to surprising results. Electrical phenomena. First it was shown that there are many other bodies which will act in like manner; then came the invention of the electrical machine, the discovery of electrical repulsion, and the spark; the differences of conductibility in bodies; the apparently two species of electricity, vitreous and resinous; the general law of attraction and repulsion; the wonderful phenomena of the Leyden phial and the electric shock; the demonstration of the identity of lightning and electricity; the means of protecting buildings and ships by rods; the velocity of electric movement—that immense distances can be passed through in an inappreciable [377] time; the theory of one fluid and that of two; the mathematical discussion of all the phenomena, first on one and then on the other of these doctrines; the invention of the torsion balance; the determination that the attractive and repulsive forces follow the law of the inverse squares; the conditions of distribution on conductors; the elucidation of the phenomena of induction. Voltaic electricity. At length, when discovery seemed to be pausing, the facts of galvanism were announced in Italy. Up to this time it was thought that the most certain sign of the death of an animal was its inability to exhibit muscular contraction: but now it was shown that muscular movements could be excited in those that are dead and even mutilated. Then followed quickly the invention of the Voltaic pile. Results of the discovery of Galvani. Who could have foreseen that the twitching of a frog's leg in the Italian experiments would establish beyond all question the compound nature of water, separating its constituents from one another? would lead to the deflagration and dissipation in a vapour of metals that could hardly be melted in a furnace? would show that the solid earth we tread upon is an oxide? yield new metals light enough to swim upon water, and even seem to set it on fire? produce the most brilliant of all artificial lights, rivalling if not excelling, in its intolerable splendour the noontide sun? would occasion a complete revolution in chemistry, compelling that science to accept new ideas, and even a new nomenclature? that it would give us the power of making magnets capable of lifting more than a ton, and cast a light on that riddle of ages, the pointing of the mariner's compass north and south, explain the mutual attraction or repulsion of magnetic needles? that it would enable us to form exquisitely in metal casts of all kinds of objects of art, and give workmen a means of gilding and silvering without risk to their health? that it would suggest to the evil disposed the forging of bank notes, the sophisticating of jewelry, and be invaluable in the uttering of false coinage? that it would carry the messages of commerce and friendship instantaneously across continents or under oceans, and "waft a sigh from Indus to the pole?"

[378] Yet this is only a part of what the Italian experiment, carried out by modern methods, has actually done. Could there be a more brilliant exhibition of their power, a brighter earnest of the future of material philosophy?

Discoveries in magnetism.As it had been with amber, so with the magnet. Its properties had lain uninvestigated for two thousand years, except in China, where the observation had been made that its qualities may be imparted to steel, and that a little bar or needle so prepared, if floated on the surface of water or otherwise suspended, will point north and south. In that manner the magnet had been applied in the navigation of ships, and in journeys across trackless deserts. The first European magnetical discovery was that of Columbus, who observed a line of no variation west of the Azores. Then followed the detection of the dip, the demonstration of poles in the needle, and of the law of attraction and repulsion; the magnetic voyage undertaken by the English government; the construction of general variation charts; the observation of diurnal variation; local perturbations; the influence of the Aurora, which affects all the three expressions of magnetical power; the disturbance of the horary motion simultaneously over thousands of miles, as from Kasan to Paris. In the meantime, the theory of magnetism improved as the facts came out. Its germ was the Cartesian vortices, suggested by the curvilinear forms of iron filings in the vicinity of magnetic poles. The subsequent mathematical discussion was conducted upon the same principles as in the case of electricity.

Electro-magnetism.Then came the Danish discovery of the relations of electricity and magnetism, illustrated in England by rotatory motions, and in France adorned by the electrodynamic theory, embracing the action of currents and magnets, magnets and magnets, currents and currents. The generation of magnetism by electricity was after a little delay followed by its converse, the production of electricity by magnetism; and thermoelectric currents, arising from the unequal application or propagation of heat, were rendered serviceable in producing the most sensitive of all thermometers.

Of light and optics. [379] The investigation of the nature and properties of light rivals in interest and value that of electricity. What is this agent, light, which clothes the earth with verdure, making animal life possible, extending man's intellectual sphere, bringing to his knowledge the forms and colours of things, and giving him information of the existence of countless myriads of worlds? What is this light which, in the midst of so many realities, presents him with so many delusive fictions, which rests the coloured bow against the cloud—the bow once said, when men transferred their own motives and actions to the Divinity, to be the weapon of God?

Optical discoveries.The first ascertained optical fact was probably the propagation of light in straight lines. The theory of perspective, on which the Alexandrian mathematicians voluminously wrote, implies as much; but agreeably to the early methods of philosophy, which were inclined to make man the centre of all things, it was supposed that rays are emitted from the eye and proceed outwardly, not that they come from exterior objects and pass through the organ of vision inwardly. Even the great geometer Euclid treated the subject on that erroneous principle, an error corrected by the Arabians. In the meantime the law of reflexion had been discovered; that for refraction foiled Alhazen, and was reserved for a European. Among natural optical phenomena the form of the rainbow was accounted for, notwithstanding a general belief in its supernatural origin. Its colours, however, could not be explained until exact ideas of refrangibility, dispersion, and the composition of white light were attained. The reflecting telescope was invented; the recognized possibility of achromatism led to an improvement in the refractor. Colours and white light. A little previously the progressive motion of light had been proved, first for reflected light by the eclipses of Jupiter's satellites, then for the direct light of the stars. A true theory of colours originated with the formation of the solar spectrum; that beautiful experiment led to the discovery of irrationality of dispersion and the fixed lines. The phenomena of refraction in the case of Iceland spar were examined, and the law for the ordinary and [380] extraordinary rays given. At the same time the polarization of light by double refraction was discovered. A century later it was followed by polarisation by reflexion and single refraction, depolarization, irised rings, bright and black crosses in crystals, and unannealed or compressed glass, the connexion between optical phenomena and crystalline form, uniaxial crystals giving circular rings and biaxial oval ones, and circular and elliptical polarization.

The beautiful colours of soap-bubbles, at first mixed up with those of striated and dotted surfaces, were traced to their true condition—thickness. The determination of thickness of a film necessary to give a certain colour was the first instance of exceedingly minute measures beautifully executed. These soon became connected with fringes in shadows, and led to ascertaining the length of waves of light.

Vision; the functions of the eye. Meantime more correct ideas respecting vision were obtained. Alhazen's explanation of the use of the retina and lens was adopted. This had been the first truly scientific investigation in physiology. The action of the eye was reduced to that of the camera-obscura described by Da Vinci, and the old notion of rays issuing therefrom finally abandoned. It had held its ground through the deceptive illustration of the magic-lantern. Of this instrument the name indicates the popular opinion of its nature. In the stories of necromancers and magicians of the time are to be found traces of applications to which it was insidiously devoted—the raising of the dead, spectres skipping along the ground or dancing on the walls and chimneys, pendulous images, apparitions in volumes of smoke. Optical instruments. These early instruments were the forerunners of many beautiful inventions of later times—the kaleidoscope, producing its forms of marvellous symmetry: the stereoscope, aided by photography, offering the very embodiment of external scenery; the achromatic and reflecting telescope, to which physical astronomy is so greatly indebted; and the achromatic microscope, now working a revolution in anatomy and physiology.

The undulatory theory.In its theory optics has presented a striking contrast to acoustics. Almost from the very beginning it was [381] recognized that sound is not a material substance emitted from the sounding body, but only undulations occurring in the air. For long, optics failed to reach an analogous conclusion. The advancement of the former science has been from the general principle down to the details, that of the latter from the details up to the general principle.

That light consists of undulations in an elastic medium was first inferred in 1664. Soon after, reflexion, refraction, and double refraction were accounted for on that principle. The slow progress of this theory was doubtless owing to Newton's supremacy. He gave a demonstration in the second book of the "Principia" (Prop. 42) that wave motions must diverge into the unmoved spaces, and carried popular comprehension with him by such illustrations as that we hear sounds though a mountain interpose. It was thought that the undulatory theory was disposed of by the impossibility of seeing through a crooked pipe, though we can hear through it; or that we cannot look round a corner, though we can listen round one.