History of the Intellectual Development of Europe Volume II Part 19

CHAPTER XI.

THE EUROPEAN AGE OF REASON--(_Continued_).

THE UNION OF SCIENCE AND INDUSTRY.

_European Progress in the Acquisition of exact Knowledge.--Its Resemblance to that of Greece._

_Discoveries respecting the Air.--Its mechanical and chemical Properties.--Its Relation to Animals and Plants.--The Winds.

--Meteorology.--Sounds.--Acoustic Phenomena._

_Discoveries respecting the Ocean.--Physical and chemical Phenomena.--Tides and Currents.--Clouds.--Decomposition of Water._

_Discoveries respecting other material Substances.--Progress of Chemistry._

_Discoveries respecting Electricity. Magnetism, Light, Heat._

_Mechanical Philosophy and Inventions.--Physical Instruments.--The Result ill.u.s.trated by the Cotton Manufacture.--Steam-engine.--Bleaching.

--Ca.n.a.ls.--Railways.--Improvements in the Construction of Machinery.--Social Changes produced.--Its Effect on intellectual Activity._

_The scientific Contributions of various Nations, and especially of Italy._

The Age of Reason in Europe presents all the peculiarities of the Age of Reason in Greece. There are modern representatives of King Ptolemy Philadelphus among his furnaces and crucibles; of Hipparchus cataloguing the stars; of Aristyllus and Timochares, with their stone quadrants and armils, ascertaining the planetary motions; of Eratosthenes measuring the size of the earth; of Herophilus dissecting the human body; of Archimedes settling the laws of mechanics and hydrostatics; of Manetho collating the annals of the old dynasties of Egypt; of Euclid and Apollonius improving mathematics. [Sidenote: a.n.a.logies between the Age of Reason in Europe and in Greece.] There are botanical gardens and zoological menageries like those of Alexandria, and expeditions to the sources of the Nile. The direction of thought is the same; but the progress is on a greater scale, and ill.u.s.trated by more imposing results. The exploring voyages to Madagascar are replaced by circ.u.mnavigations of the world; the revolving steam-engine of Hero by the double-acting engine of Watt; the great galley of Ptolemy, with its many banks of rowers, by the ocean steam-s.h.i.+p; the solitary watch-fire on the Pharos by a thousand light-houses, with their fixed and revolving lights; the courier on his Arab horse by the locomotive and electric telegraph; the scriptorium in the Serapion, with its shelves of papyrus, by countless printing-presses; the "Almagest" of Ptolemy by the "Principia" of Newton; and the Museum itself by English, French, Italian, German, Dutch, and Russian philosophical societies, universities, colleges, and other inst.i.tutions of learning.

[Sidenote: European progress in the acquisition of knowledge.] So grand is the scale on which this cultivation of science has been resumed, so many are those engaged in it, so rapid is the advance, and so great are the material advantages, that there is no difficulty in appreciating the age of which it is the characteristic. The most superficial outline enables us to recognize at once its resemblance to that period of Greek life to which I have referred. To bring its features into relief, I shall devote a few pages to a cursory review of the progress of some of the departments of science, selecting for the purpose topics of general interest.

First, then, as respects the atmosphere, and the phenomena connected with it.

[Sidenote: The atmosphere.] From observations on the twilight, the elasticity of aerial bodies, and the condensing action of cold, the conclusion previously arrived at by Alhazen was established, that the atmosphere does not extend unlimitedly into s.p.a.ce. Its height is considered to be about forty-five miles. From its compressibility, the greater part of it is within a much smaller limit; were it of uniform density, it would not extend more than 29,000 feet. Hence, comparing it with the dimensions of the earth, it is an insignificant aerial sh.e.l.l, in thickness not the eightieth part of the distance to the earth's centre, and its immensity altogether an illusion. It bears about the same proportion to the earth, that the down upon a peach bears to the peach itself.

A foundation for the mechanical theory of the atmosphere was laid as soon as just ideas respecting liquid pressures, as formerly taught by Archimedes, were restored, the conditions of vertical and oblique pressures investigated, the demonstration of equality of pressures in all directions given, and the proof furnished that the force of a liquid on the bottom of a vessel may be very much greater than its weight.

[Sidenote: Its mechanical relations.] Such of these conclusions as were applicable were soon transferred to the case of aerial bodies. The weight of the atmosphere was demonstrated, its pressure ill.u.s.trated and measured; then came the dispute about the action of pumps, and the overthrow of the Aristotelian doctrine of the horror of a vacuum.

Coincidently occurred the invention of the barometer, and the proof of its true theory, both on a steeple in Paris and on a mountain in Auvergne. The invention of the air-pump, and its beautiful ill.u.s.trations of the properties of the atmosphere, extended in a singular manner the taste for natural philosophy.

[Sidenote: Its chemical relations.] The mechanics of the air was soon followed by its chemistry. From remote ages it had been numbered among the elements, though considered liable to vitiation or foulness. The great discovery of oxygen gas placed its chemical relations in their proper position. One after another, other gases, both simple and compound, were discovered. Then it was recognized that the atmosphere is the common receptacle for all gases and vapours, and the problem whether, in the course of ages, it has ever undergone change in its const.i.tution arose for solution.

[Sidenote: The antagonism of animals and plants.] The negative determination of that problem, so far as a few thousand years are concerned, was necessarily followed by a recognition of the antagonism of animals and plants, and their mutually balancing each other, the latter accomplis.h.i.+ng their duty under the influence of the sun, though he is a hundred millions of miles distant. From this it appeared that it is not by incessant interventions that the sum total of animal life is adjusted to that of vegetable, but that, in this respect, the system of government of the world is by the operation of natural causes and law, a conclusion the more imposing since it contemplates all living things, and includes even man himself. The detail of these investigations proved that the organic substance of plants is condensed from the inorganic air to which that of all animals returns, the particles running in ever-repeating cycles, now in the air, now in plants, now in animals, now in the air again, the impulse of movement being in the sun, from whom has come the force incorporated in plant tissues, and eventually disengaged in our fires, s.h.i.+ning in our flames, oppressing us in fevers, and surprising us in blushes.

[Sidenote: The winds; their origin and nature.] Organic disturbances by respiration and the growth of plants being in the lowest stratum of the air, its uniformity of composition would be impossible were it not for the agency of the winds and the diffusion of gases, which it was found would take place under any pressure. The winds were at length properly referred to the influence of the sun, whose heat warms the air, causing it to ascend, while other portions flow in below. The explanation of land and sea breezes was given, and in the trade-wind was found a proof of the rotation of the earth. At a later period followed the explanation of monsoons in the alternate heating and cooling of Asia and Africa on opposite sides of the line, and of tornadoes, which are disks of air rotating round a translated axis with a diameter of one hundred or one hundred and fifty miles, the axis moving in a curvilinear track with a progressive advance of twenty or twenty-five miles an hour, and the motions being in opposite directions in opposite hemispheres of the globe.

The equatorial calms and trade-winds accounted for on physical principles, it was admitted that the winds of high lat.i.tudes, proverbially uncertain as they are, depend in like manner on physical causes.

With these palpable movements there are others of a less obvious kind.

Through the air, and by reason of motions in it, sounds are transmitted to us.

[Sidenote: Of sounds; their velocity.] The Alexandrian mathematicians made sound a favourite study. Modern acoustics arose from the recognition that there is nothing issuing from the sounding body, but that its parts are vibrating and affecting the medium between it and the ear. Not only by the air-pump, but also by observations in the rare atmosphere of the upper regions, it was shown that the intensity of sound depends upon the density. On the top of a mountain the report of a pistol is no louder than that of a cracker in the valley. As to the gradual propagation of sounds, it was impossible to observe fire-arms discharged at a distance without noticing that the flash appears longer before the report in proportion as the distance is greater. The Florentine academicians attempted a determination of the velocity, and found it to be 1148 feet in a second. More accurate and recent experiments made it 108942 feet at the freezing-point of water; but the velocity, though independent of the density, increases with the temperature at the rate of 114 foot for each degree. For other media the rate is different; for water, about 4687 feet in a second, and in cast iron about 10-1/2 times greater than in air. All sounds, irrespective of their note or intensity, move at the same velocity, the medium itself being motionless in the ma.s.s. No sound can pa.s.s through a vacuum. The sudden aerial condensation attending the propagation of a sound gives rise to a momentary evolution of heat, which increases the elasticity of the air, and hence the velocity is higher than 916 feet in a second, otherwise the theoretical rate.

[Sidenote: Acoustic phenomena.] Turning from soniferous media to sounding bodies, it was shown that the difference between acute and grave sounds depends on the frequency of vibration. The ear can not perceive a sound originating in less than thirty-two vibrations in a second, nor one of more than 24,000. The actual number of vibrations in a given note was counted by means of revolving wheels and other contrivances. I have not s.p.a.ce to relate the investigation of many other acoustic facts, the reference of sounds to phases of condensation, and rarefaction in the elastic medium taking place in a normal direction; the affections of note, intensity, quality; the pa.s.sage in curved lines and around obstacles; the production of sympathetic sounds; nodal points; the effect of reeds; the phenomena of pipes and flutes, and other wind instruments; the various vibrations of solids, as bells; or of membranes, as drums; visible acoustic lines; the reflexion of undulations by surfaces of various forms; their interferences, so that, no matter how intense they may be individually, they can be caused to produce silence; nor of whispering galleries, echoes, the nature of articulate sounds, the physiology of the vocal and auditory organs of man, and the construction of speaking machines.

[Sidenote: The ocean; its size.] Like the air, the ocean, which covers three-fourths of the earth's surface, when reduced to a proper standard of measure, loses very much of its imposing aspect. The varnish that covers a twelve-inch globe represents its relative dimension not inadequately.

[Sidenote: Tides and currents.] On the theory of gravitation, the tides of the ocean were explained as depending on the attractive force of the sun and moon. Its currents, in a general manner, are a.n.a.logous to those of the air. They originate in the disturbing action of solar heat, the temperature of the sea varying from 85 in the torrid zone to the freezing-point as the poles are approached. Its specific gravity at the equator is estimated at 1028; but this density necessarily varies with the rate at which superficial evaporation takes place; the pure vapour rising, leaves a more concentrated salt solution. The effect is therefore, in some degree, to counteract the expansion of the water by warmth, for the sun-rays, being able to penetrate several feet below the surface, correspondingly raise the temperature of that portion, which expands and becomes lighter; but, simultaneously, surface evaporation tends to make the water heavier. Notwithstanding this, currents are established through the preponderance of the dilatation, and of them the Gulf Stream is to us the most striking example.

[Sidenote: Effects of ocean streams.] The physical action of the sun-rays in occasioning currents operates through the expansion of water, of which warm portions ascend to the surface, colder portions from beneath setting in to supply their place. These currents, both hot and cold, are affected by the diurnal rotation of the earth, the action being essentially the same as that for the winds. They exert so great an influence as conveyers of heat that they disturb the ordinary climate relation depending on the sun's position. In this way the Gulf Stream, a river of hot water in a sea of cold, as soon as it spreads out on the surface of the Atlantic in higher lat.i.tudes, liberates into the air the heat it has brought from the torrid zone; and this, being borne by the south-west wind, which blows in those localities for the greater part of the year, to the westerly part of the European continent, raises by many degrees the mean annual temperature, thus not only regulating the distribution of animals and plants, but also influencing human life and its pursuits, making places pleasant that would otherwise be inclement, and even facilitating the progress of civilization. Whatever, therefore, can affect the heat, the volume, the velocity, the direction of such a stream, at once produces important consequences in the organic world.

[Sidenote: Physical and chemical relations of water.] The Alexandrian school had attained correct ideas respecting the mechanical properties of water as the type of liquids. This knowledge was, however, altogether lost in Europe for many ages, and not regained until the time of Stevinus and Galileo, who recovered correct views of the nature of pressure, both vertical and oblique, and placed the sciences of hydrostatics and hydrodynamics on exact foundations. The Florentine academicians, from their experiments on water inclosed in a globe of gold, concluded that it is incompressible, an error subsequently corrected, and its compressibility measured. The different states in which it occurs, as ice, water, steam, were shown to depend altogether on the amount of latent heat it contains. Out of these investigations originated the invention of the steam-engine, of which it may be said that it has revolutionized the industry of the world. Soon after the explanation of the cause of its three states followed the great discovery that the opinion of past ages respecting its elementary nature is altogether erroneous. It is not a simple element, but is composed of two ingredients, oxygen and hydrogen, as was rigorously proved by decomposing and forming it. By degrees, more correct views of the nature of evaporation were introduced; gases and vapours were found to coexist in the same s.p.a.ce, not because of their mutual solvent power, but because of their individual and independent elasticity. The instantaneous formation of vapours in a vacuum showed that the determining condition is heat, the weight of vapour capable of existing in a given s.p.a.ce being proportional to the temperature. More scientific views of the nature of maximum density were obtained, and on these principles was effected the essential improvement of the low pressure steam-engine--the apparent paradox of condensing the steam without cooling the cylinder.

In like manner much light was cast on the meteorological functions of water. It was seen that the diurnal vaporization from the earth depends on the amount of heat received, the vapour rising invisibly in the air till it reaches a region where the temperature is sufficiently low.

There condensation into vesicles of perhaps 1/50000 of an inch in diameter ensues, and of myriads of such globules a cloud is composed.

[Sidenote: Clouds and their nomenclature.] Of clouds, notwithstanding their many forms and aspects, a cla.s.sification was given--cirrus, c.u.mulus, stratus, etc. It was obvious why some dissolve away and disappear when they encounter warmer or drier s.p.a.ces, and why others descend as rain. It was shown that the drops can not be pure, since they come in contact with dust, soluble gases, and organic matter in the air.

[Sidenote: The return of water to the sea.] Sinking into the ground, the water issues forth as springs, contaminated with whatever is in the soil, and finds its way, through streamlets and rivers, back to the sea, and thus the drainage of countries is accomplished. Through such a returning path it comes to the receptacle from which it set out; the heat of the sun raised it from the ocean, the attraction of the earth returns it thereto; and, since the heat-supply is invariable from year to year, the quant.i.ty set in motion must be the same. Collateral results of no little importance attend these movements. Every drop of rain falling on the earth disintegrates and disturbs portions of the soil; every stream carries solid matter into the sea. It is the province of geology to estimate the enormous aggregate of detritus, continents washed away and new continents formed, and the face of the earth remodelled and renewed.

[Sidenote: Progress of chemistry.] The artificial decomposition of water const.i.tutes an epoch in chemistry. The European form of this science, in contradistinction to the Arabian, arose from the doctrine of acids and alkalies, and their neutralization. This was about A.D. 1614. It was perceived that the union of bodies is connected with the possession of opposite qualities, and hence was introduced the idea of an attraction of affinity. On this the discovery of elective attraction followed. Then came the recognition that this attraction is connected with opposite electrical states, chemistry and electricity approaching each other. A train of splendid discoveries followed; metals were obtained light enough to float on water, and even apparently to accomplish the proverbial impossibility of setting it on fire. In the end it was shown that the chemical force of electricity is directly proportional to its absolute quant.i.ty. [Sidenote: Attraction. The elements.] Better views of the nature of chemical attraction were attained, better views of the intrinsic nature of bodies. The old idea of four elements was discarded, as also the Saracenic doctrine of salt, sulphur, and mercury. The elements were multiplied until at length they numbered more than sixty.

[Sidenote: Theory of phlogiston.] Alchemy merged into chemistry through the theory of phlogiston, which accounted for the change that metals undergo when exposed to the fire on the principle that something was driven off from them--a something that might be restored again by the action of combustible bodies. It is remarkable how adaptive this theory was. It was found to include the cases of combustive operations, the production of acids, the breathing of animals. It maintained its ground even long after the discovery of oxygen gas, of which one of the first names was dephlogisticated air.

But a false theory always contains within itself the germ of its own destruction. The weak point of this was, that when a metal is burnt the product ought to be lighter than the metal, whereas it proves heavier.

[Sidenote: Introduction of the balance into chemistry.] At length it was detected that what the metal had gained the surrounding air had lost.

This discovery implied that the balance had been resorted to for the determination of weights and for the decision of physical questions. The reintroduction of that instrument--for, as we have seen, it had ages before been employed by the Saracen philosophers, who used several different forms of it--marked the epoch when chemistry ceased to be exclusively a science of quality and became one of quant.i.ty.

[Sidenote: Theory of oxygen, and the nomenclature.] On the ruins of the phlogistic theory arose the theory of oxygen, which was sustained with singular ability. Its progress was greatly facilitated by the promulgation of a new nomenclature in conformity to its principles, and of remarkable elegance and power. In the course of time it became necessary, however, to modify the theory, especially by deposing oxygen from the att.i.tude of sovereignty to which it had been elevated, and a.s.signing to it several colleagues, such as chlorine, iodine, etc. The introduction of the balance was also followed by important consequences in theoretical chemistry, among which pre-eminently was the establishment of the laws of combinations of bodies.

[Sidenote: Present state of chemistry.] Extensive and imposing as is the structure of chemistry, it is very far from its completion. It is so surrounded by the scaffolding its builders are using, it is so deformed with the materials of their work, that its true plan can not yet be made out. In this respect it is far more backward than astronomy. It has, however, disposed of the idea of the destruction and creation of matter.

[Sidenote: Indestructibility of matter.] It accepts without hesitation the doctrine of the imperishability of substance; for, though the aspect of a thing may change through decompositions and recombinations, in which its const.i.tuent parts are concerned, every atom continues to exist, and may be recovered by suitable processes, though the entire thing may have seemingly disappeared. A particle of water raised from the sea may ascend invisibly through the air, it may float above us in the cloud, it may fall in the rain-drop, sink into the earth, gush forth again in the fountain, enter the rootlets of a plant, rise up with the sap to the leaves, be there decomposed by the sunlight into its const.i.tuent elements, its oxygen and hydrogen; of these and other elements, acids and oils, and various organic compounds may be made: in these or in its undecomposed state it may be received in the food of animals, circulate in their blood, be essentially concerned in acts of intellection executed by the brain, it may be expired in the breath.

Though shed in the tear in moments of despair, it may give birth to the rainbow, the emblem of hope. Whatever the course through which it has pa.s.sed, whatever mutations it has undergone, whatever the force it has submitted to, its elementary const.i.tuents endure. Not only have they not been annihilated, they have not even been changed; and in a period of time, long or short, they find their way as water back again to the sea from which they came.

[Sidenote: Electrical discoveries.] Discoveries in electricity not only made a profound impression on chemistry, they have taken no insignificant share in modifying human opinion on other very interesting subjects. In all ages the lightning had been looked upon with superst.i.tious dread. The thunderbolt had long been feigned to be the especial weapon of Divinity. A like superst.i.tious sentiment had prevailed respecting the northern lights universally regarded in those countries in which they display themselves as glimpses of the movements of the angelic host, the banners and weapons of the armies of heaven. A great blow against superst.i.tion was struck when the physical nature of these phenomena was determined. As to the connexion of electrical science with the progress of civilization, what more needs to be said than to allude to the telegraph?

[Sidenote: Theories of electricity.] It is an ill.u.s.tration 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.

[Sidenote: 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 ident.i.ty of lightning and electricity; the means of protecting buildings and s.h.i.+ps by rods; the velocity of electric movement--that immense distances can be pa.s.sed through in an inappreciable 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. [Sidenote: 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.

[Sidenote: 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 const.i.tuents 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 compa.s.s 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 friends.h.i.+p instantaneously across continents or under oceans, and "waft a sigh from Indus to the pole?"

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?

[Sidenote: 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 s.h.i.+ps, 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.

[Sidenote: Electro-magnetism.] Then came the Danish discovery of the relations of electricity and magnetism, ill.u.s.trated 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.

[Sidenote: Of light and optics.] 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 G.o.d?

[Sidenote: 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 pa.s.s 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. 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 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 gla.s.s, 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.