clouds—fire-mist—vapour of platinum—components of chalk—constituents of the prim?val fire-mist—objections—origin of the mist—remarkable discovery of the century—analysis of the sun—spectroscopic analysis—simplicity of solar chemistry—potassium—a drop of water—the solar elements—calcium—the most important lines in the solar spectrum—photograph of the sun—carbon in the solar clouds—function of carbon—bunsen’s burner illustrates carbon in the sun—carbon vapours in the sun—the supposed limit to our knowledge of the heavens—characteristics of spectroscopic work—bearing on the nebular theory.

in considering how the formation of our solar system was brought about, we naturally first enquire as to the material of which this superb scheme is constructed. what were the materials already to hand from which, in pursuance of the laws of nature, the solar system was evolved?

see the robust and solid nature of this earth of ours, and the robust and solid nature of the moon and the planets. it might at first sight be concluded that the primitive materials of our earth had also been in the solid state. but such is not the case. the primitive material of the solar system was not solid, it was 262not even liquid. what we may describe as the mother-substance of the universe must have been of quite a different nature; we can give an illustration of the physical character of that substance.

the lover of nature delights to look at the mountains and the trees, the lakes and the rivers. but he will not confine his regard merely to the objects on the earth’s surface. he, no less than the artist and the poet, delights to gaze at that enchanting scenery which, day by day, is displayed in infinite beauty overhead; that scenery which is not wholly withheld even from observers whose lives may be passed amid the busy haunts of men, that scenery which is so often displayed on fine days at all seasons. we are alluding to those clouds which add the charm of infinite variety to the sky above us.

it is necessary for us now to think of matter when it possesses neither the density of a solid, nor the qualities of a liquid, but rather when it has that delicate texture which the clouds exhibit. the prim?val material from which the solar system has been evolved is of a texture somewhat similar to that of the clouds. this prim?val material is neither solid nor liquid; it is what we may describe as vapour.

but having pointed to the clouds in our own sky as illustrating, in a sense, the texture of this original mother-substance of the solar system, we can carry the analogy no further. those dark and threatening masses which forbode the thunderstorm, or those beautiful fleecy clouds which enhance the loveliness of a summer’s day, are, of course, merely the vapours of water. but the vapours in the mother-substance from which systems have been evolved were by no means the vapours of 263water. they were vapours of a very different character—vapours that suggest the abodes of pluto rather than the gentle rain that blesses the earth. in the mother-substance of the solar system vapours of a great variety of substances were blended. for in the potent laboratory of nature every substance, be it a metal or any other element, or any compound, no matter how refractory, will, under suitable circumstances, be dissolved into vapour.

take, for instance, such a material as platinum. could anything be less like a vapour than this silvery metal? we know that platinum is the densest of all the elements. we know that platinum, more effectually than other metals, resists liquefaction from the application of heat. no ordinary furnace can fuse platinum; yet in another way we can overcome the resistance of this metal. the electric arc, when suitably managed, yields a temperature higher than that of any furnace. let the electric current spring from one pole of platinum to another, and a brilliant arc of light is produced by the glowing gas, which is characteristic of platinum. the light dispensed from that arc is different from the light that would be radiated if the poles were of any material other than platinum. some of the platinum has not alone been melted, it has actually been turned into vapour by the overpowering heat to which it has been subjected. thus the solidity of this substance, which resists so stubbornly the action of lower temperatures, can be overcome, and the very densest of all metals is dissolved into wisps of vapour.

we choose the case of platinum as an illustration because it is a substance exceptionally dense and exceptionally refractory. if platinum can be vaporised, 264there is not much difficulty in seeing that other elements must be capable of being vaporised also. in fact, given such heat as is found abundantly in natural sources, there is no known element, or combination of elements, which will not assume the form of gas or vapour or cloud.

at the temperature of the sun a drop of water would be forthwith resolved into its component gases of oxygen and hydrogen. in like manner a piece of chalk, if exposed to the sun, would be speedily transformed; it would first be heated red-hot and then white-hot; it is, indeed, white-hot chalk that gives us that limelight which we know so well. but the heat of the sun is far greater than the temperature of the incandescent lime. the lime would not only be heated white-hot by contact with solar heat, but still further stages would be reached. it would suffer decomposition. it would break up into three different elements: there would be the metal which we call calcium, there would be oxygen, and there would be carbon. owing to the tremendous temperature of the sun the metal would not remain in the metallic form; it would not be even in a liquid form; it would become a gas. the elements which unite to form this chalk would be not only decomposed, but they would be vaporised. what is thus stated about the drop of water and the chalk may, so far as we know, be stated equally with regard to any other compounds. it matters not how close may be the chemical association in which the elements are joined: no matter how successfully those compounds may resist the decomposition under the conditions ordinarily prevailing on earth, they have to yield 265under the overwhelming trial to which the sun would subject them. though there are many elements in the solar chemistry, there are no compounds. at the exalted temperature to which they are exposed in the sun the elements are indisposed for union with the other elements there met with, and which are at the same temperature. in these circumstances, they successfully resist all alliances.

until the last few years no elements were known in our terrestrial experience which possessed at ordinary temperatures the same qualities of resolute isolation which all elements seem to display at extreme temperatures. the famous discovery of argon, and of other strange gases associated with argon in the atmosphere and elsewhere, has revealed, to the astonishment of chemists and to the great extension of knowledge, that we have with us here elements which resist all solicitations to enter into chemical union with other substances. it is doubtless in consequence of this absolute refusal to unite that, in spite of their abundance and their wide distribution, these elements have eluded detection for centuries. to the astronomer argon is both interesting and instructive. it shows us an element which possesses, at the ordinary temperatures of the surface of the earth, a property which is true of all elements when subjected to such temperatures as are found in the sun.

think of the rocks which form the earth’s crust and of the minerals which lie far below. think of the soil which lies on its surface, of the forests which that soil supports, and the crops which it brings forth. think of the waters of the ocean, and the ice of the poles. think of the objects of every kind on this 266globe. think of the stone walls of a great building, of the iron used to give it strength, of the slates which cover it, and of the timber which forms its floors; think of the innumerable other materials which have gone towards its construction; think even of the elementary substances which go to form the bodies of animals, of the lime in their bones, and of the carbon which is so intimately associated with life itself. the nebular theory declares that those materials have not always been in the condition in which we now see them; that there was a time in which they were so hot that they were not in the solid state; they were not even in the fluid state, but were all in rolling volumes of glowing vapour which formed the great prim?val fire-cloud.

we must understand the composite nature of the primitive fire-mist from which our solar system originated. let me illustrate the matter thus: we shall suppose that a heterogeneous collection of substances is brought together, the items of which may be somewhat as follows: let there be many tons of iron and barrels of lime, some pieces of timber, and cargoes of flint; let there be lead and tin and zinc, and many other metals, from which copper and silver and several of the rarest metals must not be excluded; let there be innumerable loads of clay, which shall represent aluminium and silicon, and hogsheads of sea-water to supply oxygen, hydrogen, and sodium. there should be also, i need hardly add, many other elements; but there is no occasion to mention more; indeed, it would be impossible to give a list which would be complete.

suppose that this diverse material is submitted to a heat as intense as the most perfect furnace can make it. 267let the heat be indeed as great as that which we can get from the electric arc, or even greater still. let us suppose this heat to be raised to such a point that, not only have the most refractory metals been transformed into vapour, but the elements which were closely in combination have also been rent asunder. this we know will happen when compound substances are raised to a very high temperature. we shall suppose that the heat has been sufficient to separate each particle of water into its constituent atoms of oxygen and hydrogen; we shall suppose that the heat has been sufficient to decompose even lime itself into its constituent parts, and exhibit them in the form of vapour. the heat is to be so great that even carbon itself, the most refractory of substances, has had to yield, so that after passing through a stage of dazzling incandescence it has melted and ultimately dissolved into vapour. next let us suppose that these several vapours are blended, though we need not assume that the separate elements are diffused uniformly throughout all parts of the cloud. let us suppose that these bodies, which contributed to form the nebula, have been employed in amounts, not to be measured in tons, or in hundreds of tons, but in a thousand millions of millions of millions of millions of tons. let the mass of vapour thus arising be expanded freely through open space. let it extend over a region which is to measure hundreds of thousands of millions of miles in length and breadth and depth. then the doctrine of the earth’s beginning, which we are striving to unfold in these lectures, declares that in a fire-mist such as is here outlined the solar system had its origin.

various objections may occur to the thoughtful 268reader when asked to accept such statements. we must do our best to meet these objections. the evidence we submit must be of an indirect or circumstantial kind. direct testimony on such a subject is from the nature of the case impossible. the actual fire-mist in which our system had its origin is a mist no longer. the material that forms the solid earth beneath our feet did once, we verily believe, float in the great prim?val fire-mist. of course we cannot show you that mist. darwin could not show the original monkeys from which it would seem the human race has descended; none the less do most of us believe that our descent has really taken the line that darwin’s theory indicates.

in connection with this subject, as with most others, it is easy to ask questions which, i think we may say, no one can answer with any confidence. it may, for instance, be asked how this vast fire-mist came into existence. if it arose from heat, how did that heat happen to be present? why was all the material in the state of vapour? what, in short, was the origin of that great prim?val nebula? here we must admit that we have proposed questions to which it is impossible for us to do more than suggest answers. as to what brought the mist into existence, as to whence the materials came, and as to whence the energy was derived which has been gradually expended ever since, we do not know anything, and, so far as i can see, we have no means of knowing. conjectures on the subject are not wanting, of course, and in a later chapter we shall discuss what may be said on this matter.

i have shown you to some extent our reasons for believing that our solar system did originate in a fire-mist and even if we are not able to explain how 269the mist itself arose, yet we do not admit that our argument as to the origin of our system is thereby invalidated. that such a fire-mist as the solar system required did once exist, must surely be regarded as not at all improbable so long as we can point to the analogous nebul? or fire-mists which exist at the present moment, and which we see with our telescopes. many of these are millions of times as great as the comparatively small fire-mist that would have evolved into our solar system.

a question has sometimes been asked as to the most important discovery in astronomy which has been made in the century that has just closed. if, by the most important discovery, we mean that which has most widely extended our knowledge of the universe, i do not think there need be much hesitation in stating the answer. it seems to me beyond doubt that the most astonishing discovery of the last century in regard to the heavenly bodies is that which has revealed the elementary substances of which the orbs of heaven are composed. this discovery is the more interesting and instructive because it has taught us that the materials of the sun, of the stars, and of the nebul? are essentially the elements of which our own earth is formed, and with which chemists had already become well acquainted.

we know, of course, that this earth, no matter how various may be the rocks and minerals which form its crust, and how infinite the variety of objects, organic and inorganic, which diversify its surface, is really formed from different combinations of about eighty different elements. there are gases like oxygen and hydrogen, there are other substances like carbon 270and sulphur, and there are metals like iron and copper. these elements are sometimes met with in their free or uncombined state, like oxygen in the atmosphere, or like gold in klondike. more frequently they are found in combination, and in such combinations the characters of the constituent elements are sometimes completely transformed. a deadly gas and a curious metal, which burns as it floats on water, most certainly renounce their special characters when they unite to form the salt on our breakfast-table. who would have guessed, if the chemist had not told him, that in every wheelbarrowful of ordinary earth there are pounds of silvery aluminium, and that marble is largely composed of an extremely rare metal, which but few people have ever seen?

until the middle of the century just completed it seemed utterly impossible to form any notion as to the substances actually present in the sun. how could anyone possibly discern them by the resources of the older chemists? it might well have been doubted whether the elements of which the sun was made were the elements of which our earth was formed, and with which ordinary chemistry had made us familiar. just as the animals and plants which met the gaze of the discoverers when they landed in the new world were essentially different from those in the old world, so it might have been supposed, with good share of reason, that this great solar orb, ninety-three million miles distant, would be composed of elements totally different from those with which dwellers on the earth had been permitted to become acquainted.

this great discovery of the last century revealed 271to us the character of the elements which constitute the sun. it also added the astonishing information that they are essentially the same elements as those of which our earth itself and all which it contains are formed.

if any one had asked in the early years of the century what those elements were which entered into the composition of the sun, the question would have been deemed a silly one; it would have been regarded as hopelessly beyond the possibility of solution, and it would have been as little likely to receive an answer as the questions people sometimes ask now as to the possible inhabitants on mars.

but about the middle of the century a new era dawned; the wonderful method of spectroscopic analysis was discovered, and it became possible to examine the chemistry of the sun. the most important result was to show that the elements which enter into the composition of the sun are the same elements which enter into the composition of the earth. the student of the solar chemistry enjoys, however, one advantage over the terrestrial chemist, if it be an advantage to have his science simplified to the utmost extent. chemistry would, however, lose its chief interest if all the elements remained as obstinately neutral as argon, and disdained alliance with all other elements. it would seem that those elements which most eagerly enter into combination here, and which resist with such vehemence our efforts to divorce them, must renounce all chemical union when exposed to the tremendous temperature of the sun.

those elements which unite with the utmost eagerness at ordinary temperatures, seem to become indifferent 272to each other when subjected to the extremes of heat and cold. potassium unites fiercely with oxygen in the most familiar of all chemical experiments. potassium is indeed a strange metal, for it is of such small density that a piece cast on a basin of water will float like a chip of wood. it has such avidity for oxygen that it will decompose the water to wrench the molecules of oxygen from those of hydrogen. the union of the metal with the gas generates such heat that the strange substance bursts into flame. this is what takes place at the ordinary temperatures in the well-known experiment of the chemical lecture-table. but at extreme temperatures the greed of potassium for oxygen abates, if it does not vanish altogether. in those excessively low temperatures at which professor dewar experiments chemical affinities languish. he has reduced oxygen to a liquid, and he tells us that “a berg of silvery potassium might float for ever untarnished on an ocean of liquid oxygen.” at the excessively high temperature of the electric arc the oxygen and the potassium, whose union has been accomplished with such vehemence, cease to possess affinity, and they separate again.

the solar chemistry seems to know no combination. if a drop of water were transferred to the sun and subjected to the heat of the solar surface, it must immediately undergo decomposition. that which was a drop of water here would not remain a drop of water there; it would be at once resolved into its component elements of oxygen and hydrogen. the considerations just given greatly simplify the search for the particular bodies which are at present in the sun. we have only to test for the presence of each of eighty elements. we have not to take account of the thousands of chemical 273combinations of which these elements are susceptible under terrestrial conditions.

we are specially indebted to the late professor henry rowland, of baltimore, for a profound study of the solar spectrum. in his great work he enumerates thirty-six elements present in the sun, and the number may be increased now by at least two. eight elements he classes as doubtful, fifteen are set down as absent from the solar spectrum, and several had not been tried. iron stands foremost among all the solar elements, so far as the number of its lines are concerned. no fewer than 2,000 lines in the spectrum of the sun are attributed to this element. at the other end of the list lead is found. there is only one line apparently due to this metal. carbon is represented by about 200 lines, and calcium by about 75. if, however, we test the significance of lines not by their number, but by their intensity, then iron no longer heads the list, its place being taken by calcium (fig. 42). among the elements which rowland sets down as not contributing any recognisable lines to the solar spectrum we may mention arsenic and sulphur, phosphorus, mercury, and gold.

of the more prominent solar elements there are two or three of such special importance that we pause to give them a little consideration. who does not remember the delight of the first occasion in childhood when he was permitted to peep into a bird’s-nest and there see a group of eggs, often so exquisitely marked or so delicately tinted? how beautiful they seemed as they lay in their cosy receptacle concealed with so much cunning! among other delightful recollections of early youth many will recall a ramble by the sea-shore. we may suppose the tide had retreated, and with other 274objects left by the sea on the gleaming sand a little cowrie shell is found. how enchanted we were with our prize! how we looked at the curious marks on its lips, and the inimitable beauty of its tints!

the shell of the hedge-sparrow and the shell cast up by the sea have another quality in common besides their beauty. they have both been fabricated from the same material. lime is of course the substance from which the bird, by some subtle art of physiology, forms those exquisite walls by which the vital part of the egg is protected. the soft organism that once dwelt in the cowrie was endowed with some power by which it extracted from the waters of the ocean the lime with which it gradually built an inimitable shell. is it an exaggeration to say that this particular element calcium, this element so excessively abundant and so rarely seen, seems to enjoy some peculiar distinction by association with exquisite grace and beauty? the white marble wrought to an unparalleled loveliness by the genius of a phidias or a canova is but a form of lime. so is the ivory on which the japanese artist works with such delicacy and refinement. whether as coral in a pacific island, as a pearl in a necklace or as a stone in the parthenon, lime seems often privileged to form the material basis of beauty in nature and beauty in art.

though lime in its different forms, in the rocks of the earth or the waters of the ocean, is one of the most ordinary substances met with on our globe, yet calcium, the essential element which goes to the composition of lime, is, as we have already said, not by any means a familiar body, and not many of us, i imagine, can ever have seen it. chemistry teaches that lime is the result of a union in definite proportions between oxygen gas and 275the very shy metal, calcium. this metal is never found in nature unless in such intimate chemical union with some other element like oxygen or chlorine, that its characteristic features are altogether obscured, and would indeed never be suspected from the mere appearance of the results of the union. to see the metal calcium you must visit a chemical laboratory where, by electrical decomposition or other ingenious process, this elusive element can be induced to part temporarily from its union with the oxygen or other body for which it has so eager an affinity, and to which it returns with such alacrity. though calcium is certainly a metal, it is very unlike the more familiar metals such as gold or silver, copper or iron. a coin might conceivably be formed out of calcium, but it would have no stability like the coins of the well-known metals. calcium has such an unconquerable desire to unite with oxygen that the unstable metal will speedily grasp from the surrounding air the vital element. unless special precautions are taken to withhold from the calcium the air, or other source from whence it could obtain oxygen, the union will most certainly take place, and the calcium will resume the stable form of lime. thus it happens that though this earth contains incalculable billions of tons of calcium in its various combinations, yet calcium itself is almost unknown except to the chemist.

it is plain that calcium plays a part of tremendous significance on this earth. i do not say that it is the most important of all the elements. it would indeed seem impossible to assign that distinction to any particular element. many are, of course, of vital importance, though there are, no doubt, certain of the rarer elements with which this earth could perhaps dispense without 276being to any appreciable extent different from what it is at present. i do not know that we should be specially inconvenienced or feel any appreciable want unsatisfied, if, let us say, the element lanthanum were to be struck out of existence; and there are perhaps certain other rare bodies among the known eighty elements, about which the same remark might be made.

fig. 42.—the h. and k. lines in the photographic solar

spectrum (higgs).

but without calcium there would neither be fertile soil for plants nor bones for animals, and consequently a world, inhabited in the same manner as our present globe, would be clearly impossible. there may be lowly organisms on this earth to which calcium is of no appreciable consequence, and it is of course conceivable that a world of living types could be constructed without the aid of that particular element which is to us so indispensable. but a world without calcium would be radically different from that world which we know, so that we are disposed to feel special interest in the important modern discovery that this same element, calcium, is abundantly distributed throughout the universe. the boldest and most striking features in 277the photograph of the solar spectrum are those due to calcium (figs. 42 and 44).

in the solar spectrum are two very broad, very dark, and very conspicuous lines, known as h and k. in every photograph of that portion of the solar spectrum which, lying beyond the extreme violet, is invisible to our eyes, though intensely active on the photographic plate, these lines stand forth so boldly as to arrest the attention more than any other features of the spectrum. it had been known that these lines were due to calcium, but there were certain difficulties connected with their interpretation. some recent beautiful researches by sir william and lady huggins have cleared away all doubt. it is now certain that the presence of these lines in the spectrum demonstrates that that remarkable element which is the essential feature of lime on this earth is also found in the sun. we have also to note that these same lines have been detected in the photographic spectra of many other bodies in widely different regions of space. thus we establish the interesting result that this particular element which plays a part so remarkable on our earth is not restricted to our globe, but is diffused far and wide throughout the universe.

perhaps the most astonishing discovery made in modern times about the sun is connected with the wonderful element, helium. so long ago as 1868 sir norman lockyer discovered, during an eclipse, that the light of the sun contained evidence of the presence in that orb of some element which was then totally unknown to chemists. this new body was not unnaturally named the sun-element, or helium. but more than a quarter of a century had to elapse before any 278chemist could enjoy the opportunity of experimenting directly upon helium. no labour could prepare the smallest particle of this substance, no money could purchase it, for at that time no specimen of the element was known to exist nearer than the sun, ninety-three million miles distant. but in 1895 an astonishing discovery was made by professor ramsay. he was examining a rare piece of mineral from norway. from this mineral, clevite, the professor extracted a little gas which was to him and to all other chemists quite unknown. but on applying the spectroscope to examine the character of the light which this gas emitted when submitted to the electric current, it yielded, to their amazement, the characteristic light of helium. thus was the sun-element at last shown to be a terrestrial body, though no doubt a rare one. the circumstances that i have mentioned make helium for ever famous among the constituents of the universe. it will never be forgotten that though from henceforth it may be regarded as a terrestrial body, yet it was first discovered, not in the earth beneath our feet, but in the far-distant sun.

in a previous picture (fig. 14) we showed a photograph of a part of the sun’s surface; this striking view displays those glowing clouds from which the sun dispenses its light and heat. these clouds form a comparatively thin stratum around the sun, the interior of which is very much darker. the layer of clouds is so thin that it may perhaps be likened to the delicate skin of a peach in comparison with the luscious interior. it is in these dazzling white clouds that we find the source of the sun’s brightness. were those clouds removed, though the sun’s diameter would not be 279appreciably reduced, yet its unparalleled lustre would be at once lessened. we use the expression “clouds” in speaking of these objects, for clouds they certainly are, in the sense of being aggregates of innumerable myriads of minute beads of some substance; but those solar clouds are very unlike the clouds of our own sky, in so far as the material of which they are made is concerned. the solar clouds are not little beads of water; they are little beads of white-hot material so dazzlingly bright as to radiate forth the characteristic brilliance and splendour of the sun. the solar clouds drift to and fro; they are occasionally the sport of terrific hurricanes; they are sometimes driven away from limited areas, and in their absence we see merely the black interior of the solar globe, which we call a sun-spot. now comes the important question as to the material present in these clouds which confers on the sun its ability to radiate forth such abundant light and heat.

the profound truth already stated, that the solar elements are the same as the terrestrial elements, greatly simplifies the search for that particular element which forms those solar clouds. as the sun is made of substances already known to us by terrestrial chemistry, and as there are no chemical compounds to embarrass us, the choice of the possible constituents of those solar clouds becomes narrowed to the list of elements experimented on in our laboratories.

we owe to dr. g. johnstone stoney, f.r.s., the discovery of the particular element which forms those fire-clouds in the sun, and confers on the presiding body of the solar system the power of being so useful to the planets which owe it allegiance. carbon is the element 280in question. i need hardly add that carbon is well known as one of the most commonplace and one of the most remarkable substances in nature. a piece of coke differs from a piece of pure carbon only by the ash which the coke leaves behind when burned. timber is principally composed of this same element, and when the timber is transformed into charcoal but little more than the carbon remains. carbon is indeed everywhere present. it is, as we have mentioned, one of the elements which enter into the composition of a piece of chalk. carbon is in the earth beneath our feet; it is in the air above us. carbon is one of the chief ingredients in our food, and it is by carbon that the heat of the body is sustained. indeed, this remarkable element is intimately connected with life in every phase. every organic substance contains carbon, and it courses with the blood in our veins. it assumes the widest variety of forms, renders the greatest diversity of services, and appears in the most widely different places. carbon is indeed of a protean character, and there is a beautiful symbol of the unique position which it occupies in the scheme of nature (fig. 43). carbon is associated not alone with articles of daily utility and of plenteous abundance, but it is carbon which forms the most exquisite gems “of purest ray serene.” the diamond is, of course, merely a specimen of carbon of absolute purity and in crystalline form. great as is the importance of carbon on this earth, it is spread far more widely; it is not confined merely to the earth, for carbon abounds on other bodies in space. the most important functions of carbon in the universe are not those it renders on this earth. it was shown by dr. stoney that this same wonderful substance is indeed a solar element of vast utility. it 281is carbon which forms the glowing solar clouds to which our very life owes its origin.

in the incandescent lamp the brilliant light is produced by a glowing filament of carbon, and one reason why we employ this element in the electric lamp, instead of any other, may be easily stated. if we tried to make one of these lamps with an iron wire, we should find that when the electric current is turned on and begins to flow through the wire, the wire will, in accordance with a well-known law, become warm, then hot, red-hot, and white-hot; but even when white-hot the wire will not glow with the brightness that we expect from one of these lamps. ere a sufficient temperature can be reached the iron will have yielded, it will have melted into drops of liquid, continuity will be broken, the circuit will be interrupted, and the lamp destroyed. we should not have been much more successful if instead of iron we had tried any other metal. even a platinum wire, though it will admit of being raised to a much higher temperature than a wire of iron or a wire of steel, cannot remain in the solid condition at the temperature which would be necessary if the requisite incandescence is to be produced.

there is no known metal, and perhaps no substance whatever, which has so high a temperature of fusion as carbon. a filament of carbon, alone among the available elements, will remain continuous and unfused while transmitting a current intense enough to produce that dazzling brilliance which is expected from the incandescent lamp. this is the reason why this particular element carbon is an indispensable material for the electrician.

modern research has now demonstrated that just as we employ carbon as the immediate agent for producing 282our beautiful artificial light, so the sun uses precisely the same element as the agent of its light and heat-giving power. in the extraordinary fervour which prevails in the interior of the sun all substances of every description must submit to be melted, nay, even to be driven into vapour. an iron poker, for instance, would vanish into iron vapour if submitted to this appalling solar furnace. even carbon itself is unable to remain solid when subjected to the intense heat prevailing in the inner parts of the sun. at that heat carbon must assume the form of gas or vapour, just as iron or the other substances which yield more readily to the application of heat.

by the help of a simple experiment we may illustrate the significance of the carbon vapours in the solar economy. let us take a bunsen burner, in which the air and gas are freely mingled before they enter into combustion. if the air and the gas be properly proportioned, the combustion is so perfect that though a great deal of heat is produced there is but little light. the gas burned in this experiment ought to be the ordinary gas of our mains, which depends for its illuminating power on the circumstance that the hydrogen, of which the gas is chiefly composed, is largely charged with carbon. the illuminating power of the gas may indeed be measured by its available richness in carbon. as it enters the burner the carbon is itself in a gaseous form. this is not, of course, on account of a high temperature. the carbon of the coal-gas is in chemical union with hydrogen, and the result is in the form of invisible gases. it is these composite gases, blended with large volumes of ordinary hydrogen, which form the illuminating gas of our mains.

283in the bunsen burner the admission of a proper proportion of air, which becomes thoroughly mixed with the coal gas, produces perfect combustion. in the act of burning, the oxygen of the air unites immediately with the gas; it combines with the hydrogen to form watery vapour, and it combines with the carbon to form gases which are the well-understood products of combustion.

suppose, now, we cut off the supply of air from the bunsen burner, which can be done in a moment by placing the hand over the ring of holes at the bottom at which the air is admitted. immediately a change takes place in the combustion. in place of the steady, hardly visible, but intensely hot flame which we had before, we have now a very much larger flame which makes a bright and flickering flare that lights up the room. if we re-admit the air at the bottom of the burner the light goes down instantly; the small, pale flame replaces it, and again the perfect combustion gives out intense heat at the expense of the light.

the remarkable change in the character of a gas-flame produced by admitting air to mix with the gas before combustion is, of course, easily explained. the chemical action takes place with much greater facility under these circumstances. the union of the carbon in the coal gas with the oxygen then takes place so thoroughly and instantaneously that the carbon never seems to have abandoned the gaseous form even for a moment in the course of the transformation. but in the case where air is not permitted to mingle with the gas, the supply of oxygen to unite with the incandescent gases can only be obtained from the exterior of the flame. the consequence is that the glowing 284gas charged with carbon vapour is chilled to some extent by contact with the cold air. it therefore seems as if the union of the hydrogen with the oxygen permitted the particles of carbon in the flame to resume their solid form for a moment. but in that solid form these particles, being at a high temperature, have a wonderful efficiency for radiation, and consequently brilliance is conferred upon the light. most of the particles of carbon speedily unite with the surrounding oxygen, and re-enter the gaseous state in a different combination. some of them, however, may escape this fate, in which case they assume the undesirable form of smoke. the bunsen lamp can thus be made to give an illustration of the fact that when carbon vapours receive a chill, the immediate effect of the chill is to transform the carbon from the gaseous form to myriads of particles in the liquid, or more probably in the solid form. in the latter state the carbon possesses a power of radiation greatly in excess of that which it possessed in the gaseous state, even though the gas may have been at a much higher temperature than the white-hot solid particles.

we can now apply these principles to the explanation of the marvellous radiation of light and heat from the great orb of day. the buoyancy of the carbon vapours is one of their most remarkable characteristics; they tend to soar upwards through the solar atmosphere until they attain an elevation considerably over that of many of the other materials in the heated vapours surrounding the great luminary. we may illustrate what happens to these carbon vapours by considering the analogous case presented in the formation of ordinary clouds in our own skins. it is true, no doubt, that 285terrestrial clouds are composed of material very different from that which enters into the solar clouds. terrestrial clouds of course arise in this way; the generous warmth of the sun evaporates water from the great oceans, and transforms it into vapour. this vapour ascends through our atmosphere, not at first as a visible cloud, but in the form of an invisible vapour. it is gradually diffused throughout the upper air, until at last particles of water, but recently withdrawn from the oceans, attain an altitude of a mile or more above the surface of the earth. a transformation then awaits this aqueous vapour. in the coldness of those elevated regions the water can no longer remain in the form of vapour. the laws of heat require that it shall revert to the liquid state. in obedience to this law the vapour collects into liquid beads, and it is these liquid beads, associated in countless myriads, which form the clouds we know so well. the same phenomenon of cloud-production is witnessed on a smaller scale in the formation of the visible puffs which issue from the funnel of a locomotive. we generally describe these rolling white volumes as steam; but this language is hardly correct. steam, properly so called, is truly as invisible as the air itself; it is only after the steam has done its work and is discharged into the atmosphere, and there receives a chill, that it becomes suddenly transformed from the purely gaseous state into clustering masses of microscopic spheres of water, and thus becomes visible.

we can now understand the transformation of these buoyant carbon vapours which soar upwards in the sun. they attain an elevation at which the fearful intensity of the solar heat has been so far abated by the cold of 286outer space that the carbon gas is not permitted to remain any longer in the form of gas; it must return to the liquid or to the solid state. in the first stage on this return the carbon gas becomes transformed, just in the same way as watery vapour ascending from the earth becomes transformed into the fleecy cloud. under the influence of its fall in temperature the carbon vapour collects into a clustering host of little beads of carbon. this is the origin of the glorious solar clouds. each particle of carbon in that magnificent radiant surface has a temperature, and consequently a power of radiation, probably exceeding that with which the filament of carbon glows in the incandescent electric arc. when we consider that millions of millions of square miles on our luminary are covered with clouds, of which every particle is so intensely bright, we shall perhaps be able to form some idea of that inimitable splendour which even across the awful gulf of ninety-three million miles transmits the indescribable glory of daylight.

we are perhaps at present living rather too close to the period itself to be able to appreciate to its full extent the greatness of that characteristic discovery made in astronomy during the century just closed, to which the present chapter relates. in the early part of the last century it might have been said—indeed, by a certain very distinguished philosopher it actually was said—that a limit could be laid down bounding the possibilities of our knowledge of the heavenly bodies. it was admitted that we might study the movements of the different orbs in vastly greater detail than had been hitherto attempted, and that we might calculate the forces to which those orbs were submitted. with the help of 287mathematical analysis we might pursue the consequences of these forces to their remote ramifications; we might determine where the various orbs were situated at inimitably remote periods in the past. we might calculate the positions which they shall attain at epochs to be reached in the illimitably remote future; we might discover innumerable new stars and worlds; and we might map down and survey the distant parts of the universe. we might even sound the depths of space and determine the distances of the more remote celestial bodies, much more distant than any of those which have already yielded their secrets; we might measure the dimensions of those bodies and determine their weights; we might add scores or hundreds to the list of the known planets; we might multiply many times the number of known nebul? and star-clusters; we might make measurements of many thousands of double stars; we might essay the sublime task of forming an inventory of the stars of the universe and compiling a catalogue in which the stars and their positions would be recorded in their millions; but, said the philosopher to whom i have referred, though you might accomplish all this, and much more in the same direction, yet there is a well-marked limit to your possible achievements; you can, he said, never expect to discover the actual chemical elements of which the heavenly bodies are composed. nobody could dispute the reasonableness of this statement at the time he made it; indeed, it seemed to be a necessary deduction from our knowledge of the arts of chemistry, as those arts were understood before the middle of the last century.

in the prosecution of his researches by the older method, the chemist could no doubt discover the different 288elements of which the body was formed. that is to say, his art enabled him to accomplish this task, provided one very essential and fundamental condition could be complied with. however accomplished the chemist of fifty years ago might have been, he would assuredly have thought that he was being mocked if asked to determine the composition of a body which was 93,000,000 miles away from him. the very idea of forming an analysis under such conditions would have been scouted as preposterous. he would naturally ask that a specimen of the body should be delivered into his hands, a specimen which he could take into his laboratory, pulverise in his mortars, place in his test-tubes, treat with his re-agents, or examine with his blowpipe. only by such methods was it then thought possible to obtain an analysis and discover the elements from which any given substance was formed.

for in the early part of this century the splendid method of spectrum analysis, that method which has revealed to us so many of the secrets of nature, had not yet come into being. when that memorable event took place it was at once perceived that the spectroscope required no actual contact with the object to be tested, but only asked to receive some of the rays of light which that object dispersed when sufficiently heated. it was obvious that this new method must be capable of an enormously enlarged application. the flame producing the vapour might be at one end of the room, while the spectroscope testing the elements in that vapour might be at the other end. this new and beautiful optical instrument could analyse an object at a distance of a hundred feet. but if applicable at a distance of a hundred feet, why not at a hundred yards, 289or a hundred miles, or a hundred million miles? why might the method not be used if the source of light were as far as the sun, or as far as a star, or even as far as the remotest nebula, whose faint gleam on the sky is all that the mightiest telescope can show.

presently another great advance was recorded. as the study of this subject progressed, it was soon found that a spectrum visible to the human eye was not always indispensable for the success of the analysis. the photographic plate, which so frequently replaces the eye in other classes of observation, has also been used to replace the eye in the use of the spectroscope. a picture has thus been obtained showing the characteristic lines in the spectrum of a celestial object. that object may have been sunk in space to a distance so tremendous that even though the light travelled at a pace sufficient to complete seven circuits of our earth in each second of time, yet the rays from the object in question may have been travelling for centuries before they reached our instrument.

however the rays of light may have become weakened in the course of that journey, they still faithfully preserve the credentials of their origin. at last the light is decomposed in the spectroscope, and the several rays, which have been so closely commingled in their long voyage of myriads of miles, are now for the first time forced to pursue different tracks; they thus reach their different destinations on the photographic plate, and they there engrave their characteristic inscriptions. nature in this operation imparts for our instruction a message which it is our business to interpret. it is true that these inscriptions are not 290always easily deciphered; many of them have not yet been understood. a portion of the solar spectrum showing many of the lines in the visible region is represented in the accompanying plate.

fig. 43.—spectrum of comet showing carbon lines.

(sir w. huggins, k.c.b.)

considering the insignificance of our earth when viewed in comparison with the millions of other orbs in the universe, considering also the stupendous distances by which the earth is separated from innumerable globes which are very much greater, it is certainly not a little astonishing to learn that the elements from which the various bodies in the universe have been composed are practically the same elements as those of which our earth is built. is not this a weighty piece of evidence in favour of the theory that earth, sun, and planets are all portions of the same prim?val nebula in which these elements were blended?

the solar spectrum.

we do not, of course, mean to affirm that the great prim?val nebula was homogeneous throughout its vast extent. the waters of ocean are not strictly the same in all places; even the atmosphere is not 291absolutely uniform. nature does not like homogeneity. the original nebula, we may well believe, was irregular in form, and denser in some places than in others. we do not suppose that if we could procure a sample of nebula in one place and another sample from the same nebula, but in a different place, say a hundred million miles distant, the two would show an identity of chemical composition; two samples of rock from different parts of the same quarry will not always be identical. but we may be assured that, in general, whatever elements are present in the nebula will be widely dispersed through its extent. if from different parts of the nebula two globes are formed by condensation, though we should not affirm, and though in fact we could not believe, that those globes would be of identical composition, yet we should reasonably expect that the elementary bodies which entered into their composition would be in substantial agreement. if one element, say iron, was abundant in one globe, we should expect that iron would not be absent from the other. thus the elements represented in one 292body should be essentially those which were represented in the other.

fig. 44.—spectrum of sun during eclipse. the two

chief lines are due to calcium.

(evershed.)

it is obvious that if the sun and the earth—to confine our attention solely to those two bodies—had originated from the prim?val nebula, they would bear with them, as a mark of their common origin, a resemblance in the elementary bodies of which they were composed. when laplace framed his theory, he had not, he could not have had, the slightest notion as to the particular elements in the sun. for anything he could tell, those elements might be absolutely different from the elements in the earth. yet, even without information on this critical point, the evidence for the nebular theory appeared to him so cogent that he gave it the sanction of his name.

it cannot be denied that if spectroscopic analysis had demonstrated that the elements in the sun were totally different from the elements in the earth a serious blow would have been dealt to the nebular theory. the collateral evidence, strong as it undoubtedly is, might hardly have withstood so damaging an admission. if, on the other hand, we find, as we actually have found, that the elements in the sun and the elements in the earth are practically identical, we obtain the most striking corroboration of the truth of the nebular theory. had kant and laplace been aware of this most significant fact, they would probably have cited it as most important testimony. they would have pointed out that the iron so abundant in the earth beneath our feet is also abundant in the sun overhead. they would, i doubt not, if they had known it, have dwelt upon the circumstance that with that element, carbon, which enters into every organic body on this 293earth, our sun is also richly supplied, and they would have hardly failed to allude to the wide distribution in space of calcium, hydrogen, and many other well-known elements.

laplace mainly based his belief in the nebular theory on some remarkable deductions from the theory of probabilities. to the consideration of these we proceed in the next three chapters. we may, however, remark at the outset that if the evidence derived from probabilities seemed satisfactory to laplace one hundred years ago, this same line of evidence, strengthened as it has been by recent discoveries, is enormously more weighty, at the present day.