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

by John William Draper

The present century finally established it through the discovery of interference, the destruction of the emission theory being inevitable when it was shown that light, interfering under certain circumstances with light, may produce darkness, as sound added to sound may produce silence—results arising from the action of undulating motion. The difficulties presented by polarization were not only removed, but that class of phenomena was actually made a strong support of the theory. The discovery that two pencils of oppositely polarized light would not interfere, led at once to the theory of transverse vibrations. Great mathematical ability was now required for the treatment of the subject, and the special consideration of many optical problems from this new point of view, as, for example, determining the result of transverse vibrations coming into a medium of different density in different directions. As the theory of universal gravitation had formerly done, so now the undulatory theory began to display its power as a physical truth, enabling geometers to foresee results, and to precede the experimenter in conclusions. Among earlier results of the [382] kind was the prediction that both the rays in the biaxial crystal topaz are extraordinary, and that circular polarization may be produced by reflexion in a rhomb of glass. The phenomena of depolarization offered no special difficulty; and many new facts, as those of elliptic polarization and conical refraction, have since illustrated the power of the theory.

The ether and its movements.Light, then, is the result of ethereal undulations impinging on the eye. There exists throughout the universe and among the particles of all bodies an elastic medium, ether. By reason of the repulsion of its own parts it is uniformly diffused in a vacuum. In the interior of refracting media it exists in a state of less elasticity compared with its density than in vacuo. Vibrations communicated to it in free space are propagated through such media by the ether in their interior. The parts of shining bodies vibrate as those of sounding ones, communicating their movement to the ether, and giving rise to waves in it. They produce in us the sensation of light. The slower the vibration, the longer the wave; the more frequent, the shorter. On wave-length colour depends. In all cases the vibrations are transverse. The undulatory movement passes onward at the rate of 192,000 miles in a second. The mean length of a wave of light is 0.0000219 of an inch; an extreme red wave is about twice as long as an extreme violet one. The yellow is intermediate. The vibrations which thus occasion light are, at a mean, 555 in the billionth of a second. As with the air, which is motionless when a sound passes through it, the ether is motionless, though traversed by waves of light. That which moves forward is no material substance, but only a form, as the waves seen running along a shaken cord, or the circles that rise and fall, and spread outwardly when a stone is thrown into water. The wave-like form passes onward to the outlying spaces, but the water does not rush forward. And as we may have on the surface of that liquid waves the height of which is insignificant, or those which, as sailors say, are mountains high in storms at sea, their amplitude thus differing, so in the midst of the ether difference of amplitude is manifested to us by difference in the intensity or brilliancy of light.

The human eye; its capabilities. [383] The human eye, exquisitely constructed as it is, is nevertheless an imperfect mechanism, being limited in its action. It can only perceive waves of a definite length, as its fellow organ, the ear, can only distinguish a limited range of sounds. It can only take note of vibrations that are transverse, as the ear can only take note of those that are normal. In optics there are two distinct orders of facts; the actual relations of light itself, and the physiological relations of our organ of vision, with all its limitations and imperfections. Light is altogether the creation of the mind. The ether is one thing, light is another, just as the air is one thing and sound another. The ether is not composed of the colours of light any more than the atmospheric air consists of musical notes.

Chemical influences of light.To the chemical agency of light much attention has in recent times been devoted. Already in photography, it has furnished us an art which, though yet in its infancy, presents exquisite representations of scenery, past events, the countenances of our friends. In an almost magical way it evokes invisible impressions, and gives duration to fleeting shadows. Moreover, these chemical influences of light give birth to the whole vegetable world, with all its varied charms of colour, form, and property, and, as we have seen in the last chapter, on them animal life itself depends.

Of heat; reflexion; refraction.The conclusions arrived at in optics necessarily entered as fundamental ideas in thermotics, or the science of heat; for radiant heat moves also in straight lines, undergoes reflexion, refraction, double refraction, polarization, and hence the theory of transverse vibrations applies to it. Heat is invisible light, as light is visible heat. Correct notions of radiation originated with the Florentine academicians, who used concave mirrors; and, in the cold-ray experiment, masses of ice of five hundred pounds weight. The refraction of invisible heat was ascertained in consequence of the invention of the thermoelectric pile. Its polarization and depolarization soon followed. Already had been demonstrated the influence of the physical state of radiant surfaces, and that the heat comes also from a little depth beneath them. Exchanges of heat. [384] The felicitous doctrine of exchanges of heat imparted true ideas of the nature of calorific equilibrium and the heating and cooling of bodies, and offered an explanation of many phenomena, as, for instance, the formation of dew. The dew, nature of. This deposit of moisture occurs after sunset, the more copiously the clearer the sky; it never appears on a cloudy night; it neither ascends from the ground like an exhalation, nor descends like a rain. It shows preferences in its manner of settling, being found on some objects before it is on others. All these singular peculiarities were satisfactorily explained, and another of the mysteries, the unaccountable wonders of the Middle Ages, brought into the attitude of a simple physical fact.

Incandescence. Physical instruments.It is impossible, in a limited space, to relate satisfactorily what has been done respecting ignition, the production of light by incandescence, the accurate measurement of the conductibility of bodies, the determination of the expansions of solids, liquids, gases, under increasing temperature, the variations of the same substance at different degrees, the heat of fluidity and elasticity, and specific heat, or to do justice to the great improvements made in all kinds of instruments—balances, thermometers, contrivances for linear and angular measures, telescopes, microscopes, spectroscopes, chronometers, aerostats, telegraphs, and machinery generally. Effect of mechanical inventions. The tendency in every direction has been to practical applications. More accurate knowledge implies increasing power, greater wealth, higher virtue. The morality of man is enhanced by the improvement of his intellect and by personal independence. Our age has become rational, industrial, progressive. In its great physical inventions Europe may securely trust. There is nothing more to fear from Arabian invasions or Tartar irruptions. The hordes of Asia could be swept away like chaff before the wind. Let him who would form a correct opinion of the position of man in the present and preceding phases of his progress reflect on the losses of Christendom in Asia and Africa, in spite of all the machinery of an Age of Faith, and the present security of Europe from every barbarian or foreign attack.

[385] From almost any of the branches of industry facts might be presented illustrating the benefits arising from the application of physical discoveries. As an example, I may refer to the cotton manufacture.

Illustration from the cotton manufacture.In a very short time after the mechanical arts were applied to the manufacture of textile fabrics, so great was the improvement that a man could do more work in a day than he had previously done in a year. That manufacture was moreover accompanied by such collateral events as actually overturned the social condition throughout Europe. Among these were the invention of the steam-engine, the canal system, the prodigious development of the iron manufacture, the locomotive, and railroads; results not due to the placemen and officers to whom that continent had resigned its annals, whose effigies encumber the streets of its cities, but to men in the lower walks of life. The assertion is true that James Watt, the instrument maker, conferred on his native country more solid benefits than all the treaties she ever made and all the battles she ever won. Arkwright was a barber, Harrison a carpenter, Brindley a millwright's apprentice.

Development of the cotton manufacture in England.By the labours of Paul or of Wyatt, who introduced the operation of spinning by rollers, a principle perfected by Arkwright; by the rotating carding-engine, first devised by Paul; by the jenny of Highs or Hargreaves; the water-frame; the mule, invented by Crompton, so greatly was the cotton manufacture developed as to demand an entire change in the life of operatives, and hence arose the factory system. The steam-engine of Watt. At a critical moment was introduced Watt's invention, the steam-engine. His first patent was taken out in 1769, the same year that Arkwright patented spinning by rollers. Watt's improvement chiefly consisted in the use of a separate condenser, and the replacement of atmospheric pressure by that of steam. Still, it was not until more than twenty years after that this engine was introduced into factories, and hence it was not, as is sometimes supposed, the cause of their wonderful increase. It came, however, at a fortunate time, nearly coincident with the invention of the dressing-machine by Radcliffe and the power-loom by Cartwright.

Bleaching by chlorine. [386] If the production of textile fabrics received such advantages from mechanics, equally was it favoured by chemistry in the discovery of bleaching by chlorine. To bleach a piece of cotton by the action of the air and the sun required from six to eight months, and a large surface of land must be used as a bleach-field. The value of land in the vicinity of great towns presented an insuperable obstacle to such uses. By chlorine the operation could be completed in the course of a few hours, and in a comparatively small building, the fibre being beautifully and permanently whitened. Calico-printing by cylinders. Nor were the chemical improvements restricted to this. Calico-printing, an art practised many thousand years ago among the Egyptians, was perfected by the operation of printing from cylinders.