Three distinct parts of the nervous system of man. In the nervous system of man our attention is therefore especially demanded by three essentially distinct parts—the spinal cord, the sensory ganglia, and the cerebrum. They are the automatic, the instinctive, the intellectual. Of the first, the spinal cord, the action is automatic; by its aid we can walk, from place to place, without bestowing a thought on our movements; by it we swallow involuntarily; by it we respire unconsciously. The second portion, the sensory ganglia, is, as we have seen, the counterpart of the cephalic ganglia of invertebrates; it is the place of reception of sensuous impressions and the seat of consciousness. To these ganglia instinct is to be referred. Their function is not at all impaired by the cerebrum superposed upon them. The third portion, the cerebrum, is anatomically distinct. It is the seat of ideas. It does not directly give rise to motions, being obliged to employ for that purpose its intermediate automatic associated apparatus. Dominating control of the latter. In this realm of ideas thoughts spring forth suggestively from one another in a perpetual train or flux, and yet the highest branch of the nervous mechanism still retains traces of the modes of operation of the parts from which it was developed. Its action is still often reflex. Reason is not always able to control our emotions, as when we laugh or weep in spite of ourselves, under the impression of some external incident. Nay, more; the inciting cause may be, as we very well know, nothing material—nothing but a recollection, an idea—and yet it is enough. But these phenomena are perhaps restricted to the first or anterior lobes of the brain, and, accordingly, we remark them most distinctly in children and in animals. As the second and third lobes begin to exercise their power, such effects are brought under control.

Progressive nervous development in the animal series.There is, therefore, a regular progression, a definite improvement in the nervous system of the animal series, the plan never varying, but being persistently carried [354] out, and thus offering a powerful argument for relationship among all those successively improving forms, an observation which becomes of the utmost interest to us in its application to the vertebrates. In the amphioxus, as has been said, the cranio-spinal axis alone exists; the Cyclostome fishes are but a step higher. In fishes the true cerebrum appears at first in an insignificant manner, a condition repeated in the early embryonic state both of birds and mammals. An improvement is made in reptiles, whose cerebral hemispheres are larger than their optic lobes. As we advance to birds, a further increase occurs; the hemispheres are now of nearly sufficient dimensions to cover over those ganglia. In the lower mammals there is another step, yet not a very great one. But from the anterior lobes, which thus far have constituted the entire brain, there are next to be developed the middle lobes. In the Rodents the progress is still continued, and in the Ruminants and Pachyderms the convolutions have become well marked. It attains its maximum in man. In the higher carnivora and quadrumana the posterior or tertiary lobes appear. The passage from the anthropoid apes to man brings us to the utmost development thus far attained by the nervous system. The cerebrum has reached its maximum organization by a continued and unbroken process of development.

It occurs again in the entire life of the globe.But that is not all. The biography of the earth, the life of the entire globe, corresponds to this progress of the individual, to this orderly relation of the animal series. Commencing with the oldest rocks that furnish animal remains, and advancing to the most recent, we recognize a continual improvement in construction, indicated by the degree of advancement of the nervous system. The earliest fishes did not proceed beyond that condition of the spinal column which is to be considered as embryonic. The Silurian and Devonian rocks do not present it in an ossified state. Fishes, up to the Carboniferous epoch, had a heterocercal tail, just as the embryos of osseous fishes of the present time have up to a certain period of their life. There was, therefore, an arrest in the old extinct forms, and an advance to a higher point in the more modern. The buckler-headed fishes of the Devonian rocks had their respiratory organs and much of their digestive apparatus in the head, and showed an approximation to the tadpoles or embryos of the frog. The crocodiles of the oolite had biconcave vertebræ, like the embryos of the recent ones which have gained the capability of making an advance to a higher point. In the geological order, reptiles make their appearance next after fishes, and this is what we should expect on the principle of an ascending nervous development. Not until long after come birds, later in date and higher in nervous advancement, capable not only of instinct, but also of intelligence. Of mammals, the first that appear are what we should have expected—the marsupials; but among the tertiary rocks, very many other forms are presented, the earlier ones, whether herbivorous or carnivorous, having a closer correspondence to the archetype than the existing ones, save in their embryonic states, the analogies occurring in such minor details as the possession of forty-four teeth. Absolute necessity of admitting transmutation of forms. The biography of the earth is thus, on the great scale, typical of individual life, even that of man, and the succession of species in the [356] progress of numberless ages is the counterpart of the transmutation of an individual from form to form. As in a dissolving view, new objects emerge from old ones, and new forms spontaneously appear without the exercise of any periodical creative act.

Life of man from infancy to maturity in accordance with his anatomy.For some days after birth the actions of the human being are merely reflex. Its cranio-spinal axis alone is in operation, and thus far it is only an automaton. But soon the impressions of external objects begin to be registered or preserved in the sensory ganglia, and the evidences of memory appear. The first token of this is perhaps the display of an attachment to persons, not through any intelligent recognition of relationship, but merely because of familiarity. This is followed by the manifestation of a liking to accustomed places and a dread of strange ones. At this stage the infant is leading an instinctive life, and has made no greater advance than many of the lower mammals; but they linger here, while he proceeds onward. He soon shows high powers of memory, the exercise of reason in the determinations of judgment, and in the adaptation of varied means to varied ends.

Such is therefore the process of development of the nervous system of man; such are the powers which consequently he successively displays. His reason at last is paramount. No longer are his actions exclusively prompted by sensations; they are determined much more by ideas that have resulted from his former experiences. While animals which approach him most closely in construction require an external stimulus to commence a train of thought, he can direct his mental operations, and in this respect is parted from them by a vast interval. The states through which he has passed are the automatic, the instinctive, the intellectual; each has its own apparatus, and all at last work harmoniously together.

Conclusions from the foregoing anatomical facts. From a consideration of the construction, development, and action of the nervous system of man, we may gain correct views of his relations to other organic beings, and obtain true psychical and metaphysical theories. There is not that homogeneousness in his intellectual structure which writers on those topics so long supposed. It is a triple mechanism. Man a member of the animal series. A gentle, a gradual, a definite development reaches its maximum in him without a breach of continuity. Parts which, because of their completion, are capable of yielding in him such splendid [358] results, are seen in a rudimentary and useless condition in organisms very far down below. On the clear recognition of this rudimentary, this useless state, very much depends. It indicates the master-fact of psychology—the fact that Averroes overlooked—that, while man agrees with inferior beings in the type of his construction, and passes in his development through transformations analogous to theirs, he differs from them all in this, that he alone possesses an accountable, an immortal soul. It is true that there are some which closely approach him in structure, but the existence of structure by no means implies the exercise of functions. In the still-born infant, the mechanism for respiration, the lungs, is completed; but the air may never enter, and the intention for which they were formed never be carried out.

His life and that of the planet alike.Moreover, it appears that the order of development in the life of individual man and the order of development in the life of the earth are the same, their common features indicating a common plan. The one is the movement of a few hours, the other of myriads of ages. This sameness of manner in their progression points out their dependence on a law immutable and universal. The successive appearance of the animal series in the endless course of time has not, therefore, been accidental, but as predetermined and as certain as the successive forms of the individual. In the latter we do not find any cause of surprise in the assumption of states ever increasing in improvement, ever rising higher and higher toward the perfection destined to be attained. We look upon it as the course of nature. Why, then, should we consider the extinctions and creations of the former as offering any thing unaccountable, as connected with a sudden creative fiat or with an arbitrary sentence of destruction?

Is it probable that the individual proceeds in his movement of development under law, that the planet also proceeds in its movements under law, but that society does not proceed under law?

Eternity and universality of that law.Man, thus, is the last term of an innumerable series of organisms, which, under the domination of law, has, in the lapse of time, been evolving. Law has controlled the inorganic world, and caused the earth to pass through various physical conditions, gently and continuously succeeding one another. The plastic forms of organic beings have been modelled to suit those changing conditions. The invariability of that law is indicated by the numberless ages through which it has been maintained, its universality by its holding good in the life of the meanest individual.

But it is only a part of sociology that we have considered, and of which we have investigated the development. Comparative sociology. In the most philosophical aspect the subject includes comparative as well as human sociology. For, though there may not be society where actions are simply reflex, there is a possibility of it where they are instinctive, as well as where they are intellectual. Its essential condition being intercommunication, there are necessarily modifications depending respectively on touch or upon the higher and more delicate senses. That is none the less society which, among insects, depends upon antennal contacts. Human society, founded on speech, sight, hearing, has its indistinct beginnings, its rudiments, very low down in the animal scale, as in the bell-like note which some of the nudibranchiate gasteropods emit, or the solitary midnight tapping with which the death-watch salutes his mate. Society resting on instinct is characterised by immobility; it is necessarily unprogressive. Society resting on intellect is always advancing.

But, for the present, declining this general examination of sociology, and limiting our attention strictly to that of humanity, we can not fail to be struck with the fact that in us the direction of evolution is altogether toward [360] the intellectual, a conclusion equally impressed upon us whether our mode of examination be anatomical or historical. The aim of Nature is not at moral, but intellectual development. Anatomically we find no provision in the nervous system for the improvement of the moral, save indirectly through the intellectual, the whole aim of development being for the sake of intelligence. Historically, in the same manner, we find that the intellectual has always led the way in social advancement, the moral having been subordinate thereto. The former hay been the mainspring of the movement, the latter passively affected. It is a mistake to make the progress of society depend on that which is itself controlled by a higher power. In the earlier and inferior stages of individual life we may govern through the moral alone. In that way we may guide children, but it is to the understanding of the adult that we must appeal. Systems of policy must be in accordance therewith. A system working only through the moral must sooner or later come into an antagonism with the intellectual, and, if it do not contain within itself a means of adaptation to the changing circumstances, it must in the end be overthrown. This was the grand error of that Roman system which presided while European civilization was developing. It assumed as its basis a uniform, a stationary psychological condition in man. Forgetting that the powers of the mind grow with the possessions of the mind, it considered those who lived in past generations as being in no respect mentally inferior to those who are living now, though our children at sixteen may have a wider range of knowledge than our ancestors at sixty. That such an imperfect system could exist for so many ages is a proof of a contemporary condition of undeveloped intellect, just as we see that the understanding of a child does not revolt against the moral suasion, often intrinsically feeble, through which we attempt to influence him. But it would be as unphilosophical to treat with disdain the ideas that have served for a guide in the earlier ages of European life, as to look with contempt on the motives that have guided us in youth. Their feebleness and incompetency are excused by their suitability to the period of life to which they are applied.

[361]But whoever considers these things will see that there is a term beyond which the application of such methods cannot be extended. The Age of Reason demands intellectual incentives for the individual. The head of a family would act unwisely if he attempted to apply to his son at twenty-one the methods he had successfully used at ten; such methods could be only rendered effective by a resort to physical compulsion. A great change in the intervening years has taken place, and ideas once intrinsically powerful can exert their influence no more. The moral may have remained unchanged; it may be precisely as it was—no better, no worse; but that which has changed is the understanding. Reasoning and inducements of an intellectual kind are now needful. An attempt to persist in an absolute system by constraint would only meet with remonstrance and derision.

And the same holds good for humanity.If it is thus with the individual, so it is likewise with humanity. For centuries nations may live under forms that meet their requirements, forms suitable to a feeble state; but it is altogether illusory to suppose that such an adaptedness can continue for ever. A critical eye discerns that the mental features of a given generation have become different from those of its ancestors. New ideas and a new manner of action are the tokens that a modification has silently taken place. Though after a short interval the change might not amount to much, in the course of time there must inevitably be exhibited the spectacle of a society that had outgrown its forms, its rules of life.

Wherever, then, such a want of harmony becomes perceptible, where the social system is incompatible with the social state, and is, in effect, an obsolete anachronism, it is plainly unphilosophical and unwise to resort to means of compulsion. No matter what the power of governments or of human authorities may be, it is impossible for them to stop the intellectual advancement, for it forces its way by an organic law over which they have no kind of control.

Influence of the sun on inorganic nature,If the light, the warmth, and other physical influences of the sun could be excluded, there would be a stagnant and icy sea encircling silent and solitary shores. But the veil once withdrawn, or the influences permitted to take effect, this night and stillness would give place to activity and change. In the morning beams of the day, the tropical waters, expanding, would follow from east to west the course of the sun, each renewed dawn renewing the impulse, and adding force to the gentle but resistless current. At one place the flowing mass would move compactly; at another, caught by accidentally projecting rocks, it would give off little eddies, expending their share of its force; or, compressed in narrow passages, it would rush impetuously along. Upon its surface myriads of momentary ripples would play, or opposing winds, called into existence by similar disturbances in the air, would force it into waves, making the shores resound with their breaking surge. Twice every day, under the conjoint influences of the sun and moon, as if the inanimate globe itself were breathing, the tide would rise and fall again upon the bosom of the deep.

The eddy, the ripple, the wave, the current, are accidental forms through which the originally imparted force is displayed. They are all expending power. Their life, if such a term can be used, is not the property of themselves, but of the ocean to which they belong.

and on organic nature.Influences which thus metaphorically give life to the sea, in reality give life to the land. Under their genial operation a wave of verdure spreads over the earth, and countless myriads of animated things attend it, each like the eddies and ripples of the sea, expending its share of the imparted force. The life of these accidental forms, through which power is being transposed, belongs, not to itself, but to the universe of which it is a part.

They constitute a series.As external circumstances change, animated forms change with them, and thus arises a series of which the members stand in a connected relation. The affiliated sequence of the external circumstances is represented in the affiliated succession of living types. From parts, or from things already existing, new parts and new things emerge, the new not being added or juxtaposed to the old, but evolved or developed from it. From the homogeneous or general, the heterogeneous or special is brought forth. A new member, fashioned in secrecy and apart, is never abruptly ingrafted on any living thing. New animal types have never been suddenly located among old ones, but have emerged from them by process of transmutation. As certainly as that every living thing must die, so must it reach perfection by passing through a succession of subordinate forms. An individual, or even a species, is only a zoological phase in a passage to something beyond. An instantaneous adult, like an immortal animal, is a physiological impossibility.

Analogies between animals and man.Man agrees with animals thus approaching him in anatomical construction in many important respects. He, too, represents a continuous succession of matter, a continuous expenditure of power. Impressions of external things are concealed in his sensory ganglia, to be presented for inspection in subsequent times, and to constitute motives of action. But he differs from them in this, that what was preparatory and rudimentary in them is complete and perfect in him. From the instrument of instinct there has been developed an instrument of intellection. In the most perfect quadrupeds, an external stimulus is required to start a train of thought, which then moves on in a determinate way, their actions indicating that, under the circumstances, they reason according to the same rules as man, drawing conclusions more or less correct from the facts offered to their notice. But, the instrument of intellection completed, it is quickly brought into use, and now results of the highest order appear. The succession of ideas is under control; new trains can be originated not only by external causes, but also by an interior, a spontaneous influence. The passive has become active. Animals remember, man alone recollects. Every thing demonstrates that the development and completion of this instrument of intellection has been followed by the super-addition of an agent or principle that can use it.

But Europe, and also the Mohammedan nations of Asia, have not received with approbation that view. The human soul. To them there is an individualized impersonation of the soul, and an expectation of its life hereafter. The animal fabric is only an instrument for its use. The eye is the window through which that mysterious principle perceives: through the ear are brought to its attention articulate sounds and harmonies; by the other organs the sensible qualities of bodies are made known. From the silent chambers and winding labyrinths of the brain the veiled enchantress looks forth on the outer world, and holds the subservient body in an irresistible spell.

Extension of these views to the nature of the world.This difference between the Oriental and European ideas respecting the nature of man reappears in their ideas respecting the nature of the world. The one sees in it only a gigantic engine, in which stars and orbs are diffusing power and running through predestined mutations. The other, with better philosophy and a higher science, asserts a personal God, who considers and orders events in a vast panorama before him.

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.

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 space. 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 shell, 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 [368] 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.

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 illustrated 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 illustrations of the properties of the atmosphere, extended in a singular manner the taste for natural philosophy.

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 constitution arose for solution.

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 latitudes, 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.

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.

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 analogous 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 1·028; 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.

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. Clouds and their nomenclature. Of clouds, notwithstanding their many forms and aspects, a classification was given—cirrus, cumulus, stratus, etc. It was obvious why some dissolve away and disappear when they encounter warmer or drier spaces, 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. 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 quantity 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.

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 attitude of sovereignty to which it had been elevated, and assigning 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.

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 superstitious dread. The thunderbolt had long been feigned to be the especial weapon of Divinity. A like superstitious 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 superstition 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?

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.

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.

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.

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.

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.

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.