EIGHTH SERIES. WINTER SESSION, 1876. 
 
 ^CHESTER SCIENCE LECTURES 
 FOR THE PEOPLE. 
 
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 WHY THE EARTH'S CHEMISTRY IS AS IT IS. 
 
 BY J. NORMAN EOCKYER, ESQ., F.R.S. 
 
 LECTURE L 
 
 In the three preceding lectures of this series, the chemical 
 constitution of the Earth has been brought before you, and 
 my part in the course, as I understand it, is to deal with 
 the bodies, so far as we know them, which people space; in 
 order that the earth’s true place in nature, so far as its 
 chemical and physical constitution is concerned, may be as¬ 
 certained, and the reasons for that constitution inquired into. 
 
 For this purpose it is necessary that I should enter at some 
 length into the constitution of those masses of matter which 
 lie beyond the earth on which we dwell, and even beyond the 
 system, and, it may be, the universe, of which we form a part. 
 And you will naturally—some of you at least-^-ask, How is 
 it possible that such knowledge as this has been attained ] 
 Prof. Roscoe, when he wished to tell you about the chemical 
 composition of the earth s crust, was enabled to bring before 
 you specimens of its different constituents, and could tell you 
 how these specimens had been handled and weighed and ex¬ 
 perimented upon in different ways in his laboratory ; but when 
 we have to deal with the chemical constitution of bodies so 
 
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 MANCHESTER SCIENCE LECTURES. 
 
 many millions of miles removed from us that we know i 
 matter of fact that the light which now enables us to see tl 
 must have left them hundreds of years ago, it is perfei 
 clear that such methods as those indicated by Dr. Roscod 
 entirely powerless. In fact some other process is thM 
 with one exception. There are certain celestial mes^H 
 come to us from time to time which we can touch ai^H 
 we can handle—I mean Meteorites, which appear 
 falling stars or aerolites; bright, beautiful objects, lilH 
 rockets which are going up to-night, and which, fortB^ 
 for science, last long enough to come down to the solid crust 
 of the earth, where they cool and where we may subsequently 
 examine them, as Dr. Roscoe has already told you. Dut with 
 this exception, it is clear to you that ordinary chemical pro¬ 
 cesses are entirely out of the question. 
 
 The progress of physical science has been in this wise : — 
 As man has grown older the earth on which we dwell has 
 dwindled down. It began as the centre of the universe ; it 
 has ended as a small mass of matter revolving Bound what 
 probably is a small star—I mean the sun. But although the 
 progress of science has been thus in a way to degrade the 
 earth, I am sure you will think with me that man’s intellect 
 has been a distinct gainer by the process ; for it is not too 
 much to say that as the earth’s place in nature has dwindled 
 down, so has man’s mental horizon been extended. That is very 
 well shown by two fundamental considerations which I must 
 bring before you in the first instance. In the year 1610, or 
 thereabouts, that is to say, about two centuries and a hal f ago, 
 thanks to the labours of men in Holland and in Italy, but 
 chiefly to the genius of the immortal Galileo, the telescope 
 was invented, and we got an untold addition to our mental 
 wealth. The skies were peopled by means of the telescope, 
 and the earth, which up to that time had been supposed 
 the centre of everything, was put in its right place; bodies 
 were observed shining millions and millions of miles away— 
 bodies which up to that time had bathed the earth with light 
 without any response from the human eye ; and what was the 
 result 1 Philosophers were enabled to class all the shining 
 orbs of heaven into two great divisions—those bodies, namely, 
 which shone like the sun with a light of their own, and those 
 which shone by borrowed light. The bodies which were found 
 to shine by borrowed light and not by any light of their 
 own, were bodies which eventually were classed together 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 3 
 
 and termed the solar system—a family of planets which go 
 round the sun, each in its proper path, each in its proper 
 time ; which are lighted up by the sun; which are warmed by 
 the sun, and to the inhabitants of which the sun is the foun¬ 
 tain of every kind of energy. We have from this classification 
 the first great grouping of celestial bodies into those which 
 shine by their own light, which, with the exception of the 
 sun, are outside the solar system; and into those which 
 shine by reflected light, which classification included allttLe 
 bodies of the solar system except the sun. Now I will throw 
 on the screen a diagram of the solar system, in order that you 
 may exactly see which these bodies are, that reflect light, and 
 in this case the light of the sun. I am very anxious 
 indeed that you should understand the importance of this 
 first classification, because the next one which I shall have 
 to bring before you will go very much further into detail. 
 We have, as representing the bodies of the solar system, 
 first of all in the centre the Sun, which shines by its own 
 light ; and next, in the order of distance from it, Mercury, 
 Venus, the Earth, Mars, a group of small planets called the 
 Asteroids ; then after them, Jupiter, Saturn, Uranus, and 
 Neptune; Neptune being the last member of the solar family, 
 so far as our knowledge at present goes. 
 
 When I call your attention to the next classification, I shall 
 no longer have to refer to the illustrious Florentine, but to 
 your own townsman, Professor Balfour Stewart, and to Pro¬ 
 fessor Stokes. Their labours have given us another and more 
 searching grouping, so to speak, by which we can go into 
 much greater detail. This grouping is no longer based on the 
 teachings of the telescope, but on the teachings of the 
 spectroscope; and here, if you will allow me, I will state 
 as briefly as may be the nature of this teaching, as you will 
 find it of extreme importance, as we go along, to understand 
 the terminology which I shall have to use. The important 
 results to which we have arrived—thanks to the work of 
 Stewart, Stokes, and others, and to the introduction of the 
 spectroscope—can be shortly stated. 
 
 So long as Dr. Boscoe was telling you about taking a speci¬ 
 men of iron and analysing it, and taking a specimen of calcium 
 and weighing it, and so on, it might not have been perfectly 
 obvious to all of you that before you could recognise the 
 existence of that calcium, and before you could see the beam 
 of the scale go up or down, according to the precise weight of 
 
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 MANCHESTER SCIENCE LECTURES. 
 
 it, a certain connection had been established between that 
 calcium, let us say, and yourself—your consciousness. But 
 when I shall have to tell of the chelnical constitution of bodies 
 thousands of millions of miles away, the necessity for some 
 connection between our eyes—our consciousness—and those 
 distant objects, will force itself upon us ; and it is important 
 therefore at the threshold that we should refer to it. These 
 distant bodies are visible to us by means of their unrest ; 
 if all the bodies in space were absolutely tranquil we should 
 never see them ; but the normal condition of everything in 
 nature is a state of most beautiful and exquisite unrest. 
 Scientific men call this a state of vibration; but we need not 
 quarrel about terms. Everything in nature, far or near, is in 
 this state of unrest, and if it were not so there would be for us 
 no external world. From every material substance, including 
 these distant worlds, the vibrations of their smallest particles 
 or of their largest masses come to us along a medium which 
 scientific men call ether, not that they know all about it, but 
 because it is necessary, in order that their work may go on at 
 all, that they should assume that there is a something in¬ 
 finitely finer than matter, and not at all like the attenuated 
 matter which pervades all space. This ether forms the high¬ 
 way along which the vibrations due to the state of unrest of 
 matter travel to our eye, and afterwards to our brain, thus 
 begetting in our consciousness the impression of the material 
 world. 
 
 Here, then, we have a vibration of the most distant mass 
 of matter in the universe communicated to our optic nerves 
 by means of this ether. How comes it that any chemical 
 knowledge can be acquired concerning these bodies 1 In this 
 way. The spectroscope tells us that when we break a mass 
 of matter down to its finest particles, or, as some people 
 prefer to call them, ultimate molecules, the vibrations of these 
 ultimate parts of each different kind of matter are absolutely 
 distinct; so that if I get the ultimate particle, say of cal¬ 
 cium, and observe its vibrations by scientific means—-what 
 those scientific means are I shall show you by and by—we 
 find that the kind of unrest of one substance—of the calcium, 
 for instance—is different from the kind of unrest or mode of 
 vibration—which is the same thing—of another substance, 
 let us say sodium. Mark well that I say when we have 
 brought these substances down to their ultimate or to almost 
 their ultimate finenesses, because until we have done so the 
 
Wave lengths according to Angstrom. 
 
 through them in succession, spectra of different elements are recorded on the same photographic plate. 
 
6 
 
 MANCHESTER SCIENCE LECTURES. 
 
 vibrations of the larger molecular aggregations are absolutely 
 powerless to tell us anything about their chemical nature, but 
 they are full of teaching as to physical conditions. 
 
 Now we know that when we bring down a substance to its 
 finest state and observe, by means of the prism, the vibrations 
 it communicates to the ether, we find that using a slit in 
 the spectroscope and making these vibrations paint differ¬ 
 ent images of the slit, we get at once just as distinct a 
 series of images of the slit for each substance as we would 
 get a distinct sequence of notes if we were playing different 
 tunes on a piano. I have here a photograph which has been 
 produced by such vibrations. I hope first to show you on the 
 screen what is called the line spectrum due to the smallest 
 particles of calcium and aluminium; and if I am successful 
 you will perfectly understand the meaning of the term line 
 spectrum. Here are the lines by which the metal calcium 
 is recognised when the vibrations of its finest particles are 
 observed by means of the spectroscope. The two central lines 
 give you also the vibrations due to aluminium. If you look 
 to the other part of the screen you will think perhaps that 
 you are dealing with a different order of phenomena alto¬ 
 gether, and in that you will be perfectly justified. In truth 
 we have here on the same screen not only the line spectra of 
 calcium and of aluminium, but what is termed the channelled- 
 space spectrum of carbon ; that is to say, while the calcium 
 and the aluminium have been driven down to their finest 
 states of separation by dissociation, the carbon has not been 
 driven down so low, and therefore we get a different kind of 
 spectrum. 
 
 These vibrations having been rendered, I hope, intelligible 
 by means of these drawings, this important consideration 
 comes into play—that whenever any element finds itself in 
 this state of fineness and therefore competent to give rise to 
 these phenomena, it will give rise to them in different degiees 
 according to certain conditions. The intensest form in which 
 they may be brought before you is observed when we employ 
 electricity. In a great many cases the vibrations may be 
 rendered very intense by heat. The heat of a furnace or of 
 gas will, for instance, in a great many cases, suffice to give us 
 these phenomena ; but to see them in all their magnificence, 
 their most extreme cases, we want the highest possible tem¬ 
 peratures, or better still, the most extreme electric energy. 
 What we get is the vibration of these particles rendered visible 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 7 
 
 to our eye by the bright images of the slit or by their bright 
 “ lines.” 
 
 But that is not the only means we have of studying these 
 states of ujirest. We can study them almost equally well 
 if, instead of dealing with the radiation of light from the par¬ 
 ticles themselves, we interpose them between us and a light- 
 source of more complicated molecular structure, and hotter or 
 more violently excited than the particles themselves. From 
 such a source the light would come to us absolutely complete, as 
 it is coming to us now from that gas; that is to say, a perfectly 
 complete gamut of waves of light, from extreme red to extreme 
 violet. I say that when we deal with these particles between 
 us and a light-source competent to give us a continuous spectrum , 
 then we find that the functions of these molecules are still the 
 same, but that their effect upon our retinas is different. They 
 are not vibrating strongly enough to give us effectively light 
 of their own, but they are eager to vibrate, and, being so, they 
 are employed, so to speak, in absorbing the light which other¬ 
 wise would come to our eyes. So that whether we observe the 
 bright spectrum of calcium or any other metal, or the absorption 
 spectrum, we get lines exactly in the same part of the chromatic 
 gamut, with the difference that when we are dealing with 
 radiation we get bright lines, and when dealing with absorp¬ 
 tion we get dark ones. 
 
 Now that being so, it will be perfectly clear to all of you 
 that we have it in our power to enormously extend the 
 inquiries started by Galileo. We need no longer be content 
 with dividing the non-terrestrial bodies into those which 
 shine by their own light and those wFich shine by reflected 
 light, but we may make a classification of this kind. We have 
 first of all those bodies which we can study by means of the 
 radiation of their molecules, that is to say bodies in which 
 the mere state of unrest, as I have ventured to call it, the 
 mere giving out of light by the molecules of which these 
 celestial masses consist, is the only thing in question. Then 
 again, we have another class in which we deal not only with 
 the radiation of the interior, but with the absorption of the 
 molecules or particles by which each body is surrounded. 
 Then we have, to come back to Galileo’s classification some¬ 
 what, those bodies which we observe by means of light which 
 they reflect to us. Then again there may be, and in fact 
 there is, a class of bodies which, although they send their light 
 to us by reflection, still make this light, so to speak, pay 
 
8 
 
 MANCHESTER SCIENCE LECTURES. 
 
 a second toll on its passage, and it comes to us reflected 
 and absorbed. 
 
 NTebul^e and Comets. 
 
 When we examine into the various bodies which people 
 the, skies, we find that among those which' can be studied 
 by means of their radiation alone there are two of the 
 very largest groups. I refer to nebulae and comets. Let 
 us first deal with the nebulae. A very small telescope indeed 
 is all that is requisite to see some of the most magnificent 
 nebulae in the heavens. I will throw on the screen Lord 
 Rosse’s drawing of the nebula of Orion; but before I do so 
 I should like to show you how the spectroscopic addition 
 to our knowledge has been secured. For this purpose I 
 can show you a drawing of the eye end of the largest tele¬ 
 scope in England at the present time, one belonging to a 
 ISTorth-countryman, Mr. Newall, of Gateshead, and you will 
 at once understand how the spectroscope has been used to aid 
 the telescope to obtain these additions to our knowledge which 
 I shall have to bring before you. Here is the eye-piece end of 
 Mr. ISTewalTs telescope, which, magnified in this way, is perhaps 
 about life-size, the object-glass being some twenty-five inches in 
 diameter, and the focal length thirty or thirty-one feet. At the 
 eye-piece end of the telescope we have attached to it at the 
 focus the spectroscope, with a number of prisms depending upon 
 the amount of light which each heavenly body which has to 
 be investigated gives out. In this drawing I have shown the 
 greatest number of prisms which are used when it is a question 
 of observing the sun ; but, as you will readily understand, 
 if instead of observing the light of the sun concentrated by 
 this enormous instrument, it is a question of observing the 
 nebulae and some of the fainter stars, then, as there is always a 
 loss of light by making it traverse through any great thickness 
 of glass, the number of prisms is much reduced ; so that you 
 may say broadly that we have the greatest possible number of 
 prisms for observing the sun, and the smallest possible 
 number of prisms, say one or two, for observing the 
 spectra of the nebulae and the spectra of the fixed stars 
 —at all events of the fainter ones. I propose, in dealing 
 both with the nebulae and with comets, first to refer to the 
 telescopic appearance of these heavenly bodies, and then 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 9 
 
 afterwards draw attention to the spectroscopic results which 
 have followed. 
 
 The question of the chemical and physical constitution of 
 the nebulae is one perhaps of the most interesting in the whole 
 range of astronomical science, and it has occupied the attention 
 of our most illustrious astronomers. Having now before you the 
 modus operandi , I will throw the drawing of the nebula, which 
 
 Fig. 2 .—The eye-piece end of the Newall refractor (of 25 inches aperture) with 
 spectroscope attached. 
 
 in these latitudes is most easily seen with the smallest instru¬ 
 ment, and which, although it can be thus seen, is nevertheless 
 one of the most magnificent objects in the whole heavens: I refer 
 to the nebula of Orion. The first thing that strikes us about 
 this nebula is its intense irregularity; there seems to be nothing 
 celestial about it. Here and there we have great waves of light 
 going along in diffuse courses from the central portion. Here 
 
10 
 
 MANCHESTER SCIENCE LECTURES. 
 
 and there we have stars surrounded by a smaller nebulosity. 
 Here again we have stars without any nebulosity at all; and 
 look where we will, we see fleecy contortions and the most 
 wondrous irregularity. Now it was not to be wondered at 
 
 that with the earliest telescopes the wildest guesses and the 
 most profound thoughts were associated with these strange 
 bodies. If we look at the works of Tycho Brahe and others, 
 before the time of the astronomers of the last century, we find 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 11 
 
 them all occupied with an attempt at understanding the physics 
 and the chemistry of these strange cosmical masses. Some were 
 content to look upon them as fire-dust; others saw in them 
 enormous star clusters so infinitely removed that each element 
 of the nebulosity might represent a single star, the single 
 star being so far away, however, that, like the Milky Way, 
 which we know to be composed of stars, to the naked eye the 
 individuality of the stars did not come out. In the last 
 century however, when Sir William Herschel with his 
 wonderful perseverance at last had succeeded in eclipsing all 
 former optical instruments by his magnificent forty-feet instru- 
 
 Fig. 4.—Nebula in process of condensation. 
 
 ment at Slough, then indeed the scientific study of nebulae 
 may be said to have commenced, and in a few years he had 
 made lists of thousands of nebulae; and his son, Sir John 
 Herschel, later went to the Cape and added thousands more. 
 Nearer our own time the magnificent instrument of Lord 
 Hosse, at Parsonstown, has added to our knowledge, so that now 
 the list of nebulae is very considerable indeed. You will ask, 
 Has the wonder connected with these strange objects been 
 reduced by extended observations? I think I shall be able 
 easily to show you that it has not been so reduced. Here, so 
 far as form goes, we get a complete absence of all form— 
 
12 
 
 MANCHESTER SCIENCE LECTURES. 
 
 absolute irregularity. Allow me to call your attention to 
 some other drawings of nebulae made by Lord Rosse, in which 
 the irregularity typified by the nebula of Orion has given 
 place to something absolutely different. Here we have an 
 approximation to form and regularity, although the regu¬ 
 larity is of different kinds. We have spiral nebulae, annular 
 nebulae, and, as I may term them, Saturnine nebulae. Note 
 well the fact that the moment we leave the extreme irregularity 
 of the nebula of Orion we always have to do with a conden¬ 
 sation of some kind or other ; in some cases with something 
 between concentric and spiral convolutions round the central 
 condensation. 
 
 Here is another series of nebulae observed in the first in¬ 
 stance by Sir John Herschel, which will intensify the classi¬ 
 fication to which I have referred. We have again spirals 
 
 Fig. 5.—Saturnine Nebula. 
 
 and double spirals. Finally, I will show you one of the most 
 magnificent spiral nebulae in the heavens from the same set of 
 drawings. You see that the central condensation is fed, as it 
 were, by spirals in all directions, some of them having con¬ 
 densations on the different branches. Now, Humboldt was 
 not in possession of all these observations which I have been 
 able to bring before you to-night, but he sums up in the first 
 volume of his Cosmos these various forms of nebulae in a very 
 effective way. This summary will well repay perusal. 
 
 Now what are the modern ideas of the constitution of these 
 strange bodies ? I have already referred to the ideas of Tycho 
 Brahe, Cassini, and the earlier observers. The work of the 
 two Herschels left it as highly probable that these nebulae 
 
WHY THE EARTH'S CHEMISTRY IS AS IT IS. 
 
 13 
 
 were really masses of cosmic dust, so to speak, or some kind of 
 gaseous or vaporous material, which took these strange forms 
 because there was nothing solid about them. But when Lord 
 Bosse, with improved optical means, investigated some of the 
 nebulae which had been called irresolvable—a name given 
 because no telescope up to that time was able to break them 
 up into separate stars—he found that his telescope did break 
 them up into distinct points of light, and then for a time in the 
 pre-spectroscopic age, as one may call it, opinion swung round, 
 and held that these nebulae simply appeared as nebulae not 
 because they did not consist of stars, but because they were so 
 far away that we could not see the separate stars of which they 
 were composed. But not many years ago Dr. Huggins—to 
 whom belongs the credit of having first turned the spectroscope 
 
 Fig. 6.—Nebula with rings. 
 
 on to the nebulae—was enabled to show beyond all shadow of 
 a doubt, that we had in the nebulae something absolutely and 
 completely distinct from stars. Dr. Huggins found, on turning 
 his spectroscope upon several of these bodies, that the spectrum 
 which he got from all of them was most characteristic. It 
 was a bright line spectrum, and there was one line which was 
 common to all of them. In the lower part of the diagram the 
 bright lines visible in the spectra of the different nebulae are 
 shown ; and below, for purposes of reference, Dr. Huggins has 
 shown the positions of various bright lines in the spectra given 
 by other substances. There, for instance, in the blue-green, 
 is the bright line due to hydrogen ; there is another line in 
 the green due to magnesium ; a line in -the yellow due to 
 sodium; and so on ; and with these points of reference we 
 
14 
 
 MANCHESTER SCIENCE LECTURES. 
 
 can easily determine the place in the spectrum of the three 
 bright lines which Dr. Huggins found to be visible in the 
 spectra of almost all the nebulae examined ; the differences 
 between nebulae and nebulae, as I understand them, being in¬ 
 dicated by the relative intensity of the lines, and the amount 
 of continuous spectrum associated with them. 
 
 Having this enormous addition to our knowledge—the fact, 
 namely, that the light given out by nebulae is perfectly distinct 
 from the light given out by stars—men of science were able 
 to study the nature of nebulae from a perfectly new stand¬ 
 point. 
 
 One of the lines of the nebulae is really coincident with a 
 line of hydrogen, or, in* other words, we have to deal with 
 hydrogen gas when we are dealing with nebulae. This has 
 
 Fig. 7. —Spectrum of the Nebulse (Huggins) compared with the bright lines of 
 certain elements. 
 
 been a most interesting and important point of departure. 
 Dr. Huggins is of opinion that the nebulae consist of masses 
 of hydrogen gas; that there is nothing solid in the nebulae, 
 that it is a mere question of incandescent hydrogen, associated 
 with something else, the chemical constitution of which has 
 not yet been thoroughly established, because Dr. Huggins, 
 with all his diligence, has not been able from his examination 
 of the other chemical elements known, to get lines correspond¬ 
 ing to those other two which we saw on the screen. But 
 that is not the only view held as to the constitution of nebulae. 
 Sir William Thomson and Professor Tait consider it ex¬ 
 tremely probable that the nebulae, instead of being masses of 
 gas, may consist of clouds of stones. How this at first seems 
 so entirely at variance with the spectroscopic result, that it is 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 15 
 
 not to be wondered at that the idea was at first considered to 
 be somewhat bizarre and strange ; but if one comes to think 
 the matter out, one finds that there is a great deal of method 
 in this strangeness, because Sir William Thomson and Pro¬ 
 fessor Tait point out that if you had a cloud of stones, each 
 one of which was in motion, and therefore liable to come 
 into collision with some other stone now and then, you would 
 get heat quite sufficient to render any circumambient gas incan¬ 
 descent ; so that the phenomena of the spectroscope could be 
 explained equally well on the assumption of a cloud of stones, 
 providing always that you could at the same time show reason¬ 
 able cause why these clouds of stones were “ banging about ” 
 in an atmosphere of hydrogen. But nebulae are not the only 
 things in the universe which these distinguished Scotch pro¬ 
 fessors imagine to be composed of clouds of stones. I think 
 it is better therefore that I should postpone the further dis¬ 
 cussion of this point until we have become acquainted with 
 the second class of bodies, which we study by means of their 
 radiation—I refer to comets. 
 
 Comets. 
 
 Now when we pass from a nebula to a comet, it is clear that 
 w^e come to a body of a perfectly different order in the celestial 
 economy. Comets are distinguished from the nebulae in many 
 ways. The nebulae, as a rule, are very far removed from us, so 
 far that we have not the least idea of the distance of any one 
 of them—we know that they are not within certain limits, and 
 that they are also at rest apparently among the stars, while 
 the comets are erratic bodies, which are now in our system, 
 and now out of it; now close to us, and now infinitely removed 
 in the depths of space. 
 
 Further, we know that the comets are not at all like the 
 planets any more than they are like the nebulae, because while 
 our planets as a rule, excepting the smaller members or minor 
 planets of the system, keep to one plane round the sun, called 
 the plane of the ecliptic, which we may liken to a racecourse, 
 round which all these planets pursue their revolutions—the 
 comets do not keep to this plane, for they are as likely to 
 dash into our system from above or below as they are to come 
 into our system on the same plane as the planets. But 
 more than this, while all the planets of our system are bound 
 together by a motion which is always in the same direction 
 
16 
 
 MANCHESTER SCIENCE LECTURES. 
 
 round the sun, when a comet comes into our system it is just 
 as likely to go round the sun in an opposite direction. So 
 that in the comets we have a complete differentiation between 
 comet and nebula on the one hand, and comet and planet on 
 the other. 
 
 I have here two or three general views which will give you 
 a rough idea of the telescopic appearances of these strange 
 visitors to our system. The peculiarity connected with comets 
 generally is a double one—they have a bright head, and they 
 have one or two or several appendages, called tails, which go 
 from the head in a certain direction. There we have a comet 
 with two tails—here with three; but these are not different 
 comets ; that is the comet which appeared in 1858, as seen at 
 two different times. These views will give you a general 
 idea of the appearance of comets, and of the way in which 
 they travel among the stars. The physical interest of comets, 
 which I shall have to call your attention to, is more inti¬ 
 mately connected with the heads than with the tails; and I 
 shall therefore hope to show you two or three more drawings, 
 in which the heads of these comets will be in question. The 
 characteristics of the heads are chiefly these—that in some 
 cases we have to deal with what are called jets. The brightest 
 point is called the nucleus of the comet, and the jets are so 
 called because they seem to shoot out from the nucleus very 
 much as the sparks shoot out of a squib. 
 
 Drawings of a comet, as seen at different times, show how 
 these jets vary in appearance and direction. Instead of jets, 
 some comets present phenomena of a very different character, 
 called envelopes, which are thrown off concentrically from the 
 nucleus. These envelopes are indicated in this drawing of a 
 comet, made by Father Secchi in Dome. These then are the 
 two physical peculiarities about the heads of comets; and 
 you will see at once that we have something perfectly dis¬ 
 tinct from the nebulae and the planets, and that one class of 
 comets is at first sight different from another. The envelopes 
 have been observed to rise from the nucleus with perfect 
 and exquisite regularity in exactly the same way that the 
 jets swing backwards and forwards. So much then for a very 
 rough telescopic idea of the phenomena of comets. 
 
 What then says the spectroscope ? I will now show you the 
 diagram which I showed you before, in order to call your atten¬ 
 tion to another part of it. Formerly I called your attention to 
 the spectrum of the nebulae. I will now call your attention 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 17 
 
 to the spectrum of comets, and I am glad to have both the 
 diagrams on the screen at the same time, because you will 
 see that the spectroscopic difference is just as great as the 
 telescopic difference. Now let me ask you to recall one of the 
 first photographs I showed you—that of the carbon spectrum 
 -—and my definition of the channelled-space spectrum. You 
 will at once recognise, I am sure, that here we have exactly 
 that same kind of channelled-space spectrum that we had 
 before. Side by side with this channelled space spectrum, 
 
 Fig. 8.—Concentric envelopes of Donati's Comet. 
 
 which is the spectrum of carbon, we have the spectrum of 
 the comet, and you will see that the family likeness is verv 
 considerable, although it is true that the brightest portions of 
 the spectrum of the comet are not absolutely coincident with 
 the brightest portions of the spectrum of carbon which Mr. 
 Huggins has drawn in the upper part of the diagram. What, 
 then, is the meaning of this spectroscopic result T It is stated 
 that if the spectroscope tells us anything, it tells us that we 
 have to deal with carbon, or with hydro carbon, as certainly in 
 the case of comets, as we had to deal with hydrogen in the case 
 of nebulae. Here then we have a definite result with regard 
 
 c 
 
18 
 
 MANCHESTER SCIENCE LECTURES. 
 
 to the brighter portions of the comets, that is to say, the 
 nucleus, the envelopes, and the jets; but how about the tail l 
 The same instrument, and the polariscope, when bronght to 
 bear on these longer appendages of comets, tell us that there 
 we have perhaps no longer to deal with hydro carbon, cer¬ 
 tainly not with hydrocarbon in a state of considerable unrest, 
 for we do not get the characteristic spectrum of hydro-carbon ; 
 we get apparently from the tails merely sun-light reflected. 
 Are we then to say that comets are built up of hydro carbon % 
 No. Here again Professors Thomson and Tait come in and 
 insist strangely enough that comets as well as nebulae are 
 masses of stones; that, in short, a comet is a bit of a nebula, 
 
 Fig. 9. -Cometary spectrum (Huggins). 
 
 differing from a nebula in this, that it is in violent motion 
 as a whole, while the nebula is apparently at rest as a whole. 
 You will find in Good • Words of a few months ago an 
 important article by Professor Tait, in which he goes into this 
 question at very great length. He shows that if we have in 
 the head of a comet a mass of stones, like a swarm of bees, 
 banging about, and, at the same time, moving in an orbit 
 around the sun, or it may be in its long path from the centre 
 of our system to the centre of another system, and the stones 
 colliding, you will get heat, and some gas will be evolved; 
 some members of the mass will be quickened, while other 
 constituents of the mass will be retarded in their motion, and 
 
WHY.THE EARTH'S CHEMISTRY IS AS IT IS. 
 
 19 
 
 that in this way you have a probably sufficient explanation 
 of the various forms which the telescope has revealed to 
 us. And then finally he goes on to show that the result 
 of these collisions would be such a smashing up of the consti¬ 
 tuents of the swarm that much finely-attenuated material would 
 be left behind, sufficient to reflect sunlight, and to give rise 
 to the phenomena of the tail. 
 
 Now, curiously enough, while these ideas have been evolving 
 themselves, a distinguished Italian astronomer, Schiaparelli, 
 had also arrived at the conclusion that comets were closely con¬ 
 nected with swarms of meteors; and he arrived at this conclu¬ 
 sion by an examination of the paths of the meteors and the paths 
 of certain comets. Astronomers now know exactly when to look 
 out for what they call a meteor-swarm, or a mass of shooting- 
 stars. They know, for instance, that on the night of the 
 20th of April they will most probably see shooting-stars 
 coming from the constellation Lyra, and these shooting-stars 
 they call the Lyriad shower. They know also that on the 
 10th and 11th of August they will see other shooting-stars, 
 this time coming from the constellation Perseus. These also 
 they call the Perseids. On Nov. 13 and 14 the Leonids may 
 be expected, that is to say, that then is the time to look out for 
 shooting stars which come from the constellation Leo. Now 
 if astronomers can tell you that on a certain night—that is 
 to say, when the earth is in a certain position in its orbit— 
 shooting-stars will appear to come from a certain part of the 
 heavens, namely, some particular constellation, it is because 
 they have become acquainted with the path of those meteors 
 round the sun. They have, in fact been able to get a very 
 concrete idea of the orbits of those meteors, in the same way 
 that we have a concrete idea of the orbit of the earth round 
 the sun. They can tell exactly where and when they cut the 
 plane of the ecliptic, and other things which I need not bring 
 before you in detail. After comets have appeared two or three 
 times, astronomers can also form an equally definite idea of 
 their orbit round the sun. Now, what Schiaparelli did was 
 this—he compared the orbits of these meteoric swarms with 
 those of some of the comets, and he found some of them iden¬ 
 tical. For instance, in the case of the shower of April 20th in 
 each year he found that there was a comet observed carefully 
 in 1861 with absolutely the same path; for those shooting- 
 stars which radiate from the constellation Perseus on 10th 
 August he found that a comet observed in 1862 had exactly 
 
20 
 
 MANCHESTER SCIENCE LECTURES. 
 
 the same path ; for the Leonids, which appear on 13th and 14th 
 November, he found that a comet, observed and calculated out 
 accurately in the year 1866, had exactly the same path. And 
 
 Fig. 10.—Diagram showing the paths of Comet I., 1868, and III., 1862, and their 
 accompanying meteor swarms, and the points at which they cut the earth s path. 
 
 then with regard to the other principal group observed on the 
 27th and 28th November, he found that the falling stars 
 seen in the constellation Andromeda on those nights hod 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 21 
 
 exactly tRe orbit as a very well-calculated and most remark¬ 
 able comet named after Biela the astronomer. Now, of course 
 this suggestion of Schiaparelli gave an enormously increased 
 interest to the investigation both of comets and meteors, and 
 when the question was thoroughly gone into it was impossible 
 to avoid the conclusion that when we have a shower of falling- 
 stars we practically are going through part of a comet; and 
 that when we see a comet in the sky we actually are seeing 
 the behaviour of a swarm of meteors at a distance. 
 
 I hope to commence my next lecture by referring to some 
 other considerations with regard to these meteorites, and then 
 to call your attention to some experiments which suggest 
 that the connection between meteors or falling-stars, comets 
 and nebulae, is of the very closest description. 
 
LECTUEE II. 
 
 In the latter part of the last Lecture I referred to the 
 chemical constitution of comets, so far as the spectroscope 
 enables us to determine this constitution ; and I endeavoured 
 to point out to you what the telescope had revealed to us with 
 reference to their physical constitution. I also again dwelt 
 on one of the great triumphs of modern astronomy— 
 namely, the discovery by Schiaparelli of the intimate connec¬ 
 tion between falling stars and comets. 
 
 Meteorites. 
 
 Now, from the falling star to the meteorite is a step so 
 small that nothing need be said about it by me to-night. The 
 difference between a falling star and a meteorite is simply this 
 —that a falling star is a small mass of matter which is 
 entirely burnt up in its passage through the higher regions 
 of our air, whereas the meteorite is a falling star big enough 
 to give us some residuum after the energetic action of 
 heat has worked its will upon it in passing through the 
 atmosphere. Observations of the rapidity with which falling 
 stars and meteorites traverse our atmosphere have shown 
 beyond a doubt that a meteorite could travel, say from 
 Manchester to London, in as many seconds as an express train 
 takes hours. You may imagine, therefore, that owing to this 
 very rapid motion through a medium—and a medium con¬ 
 stantly increasing in density—such as our air, that a consider¬ 
 able resistance is offered to the passage of the meteorite. 
 This arrested motion in process of time becomes developed 
 into what we call heat, and as a result in all cases we get 
 
WHY. THE EARTH'S CHEMISTRY IS AS IT IS. 1 23 
 
 luminous effects, with this difference, that, whereas in the case 
 of the falling star, the luminous effect is the only thing 
 we get, in other cases we get in addition to it the actual 
 descent of what we may term a celestial messenger from 
 the depths of space. Of course, having these meteorites— 
 these larger masses—falling to the earth, so that we can 
 handle them, a great deal has been learned about their chemi¬ 
 cal and physical constitution, as Dr. Hoscoe has already told 
 you. 1 need not dwell at any ; great length upon this, after 
 what Dr. Roscoe has said ; but I may state that, generically, 
 these celestial messengers may be divided into four groups. 
 We have those which are almost entirely metallic. We 
 have those which are almost entirely stony. We have those 
 in which the metallic and the stony constituents are mixed 
 in various proportions; and in a fourth, or last class, in 
 addition to the materials to which I have already referred, 
 there are to be found various combinations of hydrogen with 
 carbon, termed hydro-carbons. 
 
 We have, therefore, in the language of the meteoric chemist, 
 siderites, those which contain iron; aerolites, those which are 
 chiefly stony; siderolites, which are mixed, half stony, half 
 iron and nickel; and then again the carbonaceous group. 
 Coming from this generic chemical grouping, a word may be 
 said as to their appearance. And here, if you will read (for>I 
 cannot go into this question in any great detail) the writings 
 of Maskelyne and Sorby, you will become acquainted with 
 some most extraordinary facts and coincidences. Some of 
 the meteorites are stated to exactly resemble volcanic bombs; 
 others resemble volcanic tufa ; others again bear evidence of 
 having being subjected to actions which we know nothing 
 whatever about in this earth of ours. Mr. Sorby, for instance, 
 has gone so far, and I have no doubt perfectly justifiably, as to 
 state that in some meteorites which he has examined micro¬ 
 scopically there is evidence to show that they were formed 
 in a region where, so to speak, there was no gravity; that is 
 to say, far away from the surface of any such body as the sun 
 or our earth. When we come to the actual chemical elements 
 which these meteorites contain, we find ourselves in a region 
 where knowledge is extremely rich, compared to what it is 
 at present in the case of nebulae and comets. It is antici¬ 
 pating matters somewhat, but it is worth while to state that 
 the complete list of the metallic elements of meteorites is 
 almost identical with the complete list of metals in this list 
 
24 
 
 MANCHESTER SCIENCE LECTURES. 
 
 (Table of Elements contained in the Sun, page 31). It is 
 more than a coincidence, I think, that the chief metallic con¬ 
 stituents of meteorites are almost identical with the chief 
 metallic constituents of the sun. But strange to say, this is 
 by no means the case when we come to leave on one side the 
 metallic elements and come to the metalloidal ones, such as 
 carbon, and sulphur. Up to the present moment there is no 
 published observation of the existence of any metalloid what¬ 
 ever in the sun’s atmosphere. That does not say they do 
 not exist; but at present we know nothing definite as to 
 their existence. 
 
 Given these meteorites, and assuming them to be the 
 meteoric swarm which Schiaparelli postulates for the comets 
 —that is to say, supposing that we see first in the heavens 
 a body which we call a comet, observe it with the spectro¬ 
 scope, and get from it the spectrum of hydro-carbon ; and 
 then suppose that subsequently this very same body, con¬ 
 sisting, according to hypothesis, of a swarm of meteorites, 
 comes into our air and gives us the appearance of falling stars, 
 and probably also the occurrence of a fall of a meteorite or two, 
 —what would most probably be the source of the luminosity 1 
 As a matter of fact what we do see when these bodies enter our 
 atmosphere and are rendered incandescent by arrested motion, 
 in the manner which I have already referred to, are spectro¬ 
 scopic indications of the existence of sodium. The bright yellow 
 of a falling star is due to incandescent sodium vapour, sodium 
 being that among the elements of all meteorites which is most 
 volatile as a metal. Next after that, in the cases where 
 brilliancy is extreme, and where the yellow colour of the 
 falling star gives place to brilliant white or even to a dazzling 
 bluish white, we get added to sodium indications of magnesium. 
 And after that, in the case of falling stars brighter even than 
 those which I have already supposed, we have added to the 
 sodium and the magnesium unmistakable traces of iron vapour. 
 Now this shows us very distinctly that if—I say if—according 
 to this hypothesis, we really do get as falling stars what we 
 get as the head of a comet, the temperature of a comet is 
 much less than the temperature of the falling star while it is 
 passing through our atmosphere, because in the comet we only 
 get a temperature high enough to render the most volatile 
 constituent, hydro carbon, incandescent, whereas we have 
 passed that stage altogether when the meteorite comes into our 
 Atmosphere and the incandescence of hydro caybon is replaced 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 25 
 
 by the incandescence of magnesium, sodium, and iron vapours. 
 There is, therefore, abundant proof on this hypothesis that 
 the temperature of a falling star, when it ig passing through 
 our air, is higher than the temperature of that same mass 
 of matter when it formed part of the head of a comet. 
 
 There is one point to which I think I may be permitted to 
 draw your attention, although at present it rests merely upon 
 an unendorsed observation of my own. I thought it would be 
 worth while to try what would happen if I enclosed specimens 
 of meteorites, taken at random, in a tube from which I sub¬ 
 sequently exhausted the air by a pump. After the pumping 
 had gone on for some considerable time, of course we got 
 an approach to a vacuum; and arrangements were made 
 by means of which an electric spark could pass along this 
 apparent vacuum, and give us the spectra of the gases evolved 
 from the meteorites. Taking those precautions which are 
 generally supposed to give us a spark of low temperature, 
 and passing the current, we got a luminous effect which, on 
 being analysed by the spectroscope, gave us that same spectrum 
 of hydro-carbon which Mr. Huggins, Donati, and others have 
 made us perfectly familiar with as the spectrum of the head 
 of a comet. There then we get the atmosphere of meteorites, 
 not necessarily carbonaceous meteorites, but meteorites taken 
 at random; and this atmosphere is exactly what we get in the 
 head of a comet. 
 
 Now let me go one step further; and to take that step 
 with advantage, allow me to refer to another point, noticed in 
 the last lecture, which was this—that whereas Schiaparelli had 
 connected meteorites and falling stars with comets. Professors 
 Tait and Thomson, on the other hand, had connected comets 
 with nebulae, both of them being, according to those physicists, 
 clouds of stones. Now how was one to carry these spectroscopic 
 observations into the region of the nebulae ? A Leyden jar was 
 included in the circuit, and we had what is generally supposed 
 to be an electric current giving us a very much higher tem¬ 
 perature than we had before. What then was the spectrum 1 
 The spectrum, so far as the known lines were concerned, was 
 the spectrum which we get from the nebulae; for the hydro¬ 
 carbon spectrum, which we get from the atmospheric meteor¬ 
 ites at a low temperature, was replaced by the spectrum of 
 hydrogen ; the spectrum of hydrogen coming of course from 
 the decomposition of the hydro carbon, with the curious but at 
 present unexplained fact that we got the spectrum indications 
 
26 
 
 MANCHESTER SCIENCE LECTURES. 
 
 of hydrogen without indications of carbon. In my laboratory 
 work I have come across other curious cases in which com¬ 
 pound vapours when dissociated only gave us one spectrum at 
 a time—by which I mean that in a vapour consisting of two 
 well-known substances, under one condition we only get the 
 spectrum of one substance, and under another condition we get 
 the spectrum of the other substance alone, so in others again of 
 both combined. The evidence seems therefore—though I do 
 not profess to speak with certainty—entirely in favour of the 
 ideas of Sir William Thomson and Professor Tait on the one 
 hand, and of Schiaparelli on the other. I note this because 
 I shall have again to refer to the conclusion to be drawn 
 from it, namely, that there is probably an intimate connection 
 between nebulse, comets, meteorites, and falling stars. 
 
 The Stars. 
 
 Concluding what I have to say with regard to the first 
 great group of the heavenly bodies, namely, that group which 
 we can study by means of the radiation of light apart from 
 absorption, I will now take up the next class, consisting of 
 those bodies which we study by absorption. 
 
 What do I mean when I say those bodies which we study 
 by absorption 1 ? I mean this—that whereas in the case of 
 the radiation of light the vibrating molecules directly commu¬ 
 nicate with us and set in motion the ether which ultimately 
 comes to our eyes; in absorbing bodies, on the other hand, the 
 vibrations which we study have been set in motion not directly, 
 but by the intrinsic vibration of other bodies further from us and 
 more violently agitated than the vapours themselves, In other 
 words, if you assume a mass of matter which is competent to 
 give you every wave length of the spectrum—that is to say, 
 a continuous spectrum, and if you assume around it indi¬ 
 vidualised vapours at a lower temperature, those vapours, 
 although they cannot be studied by their radiation, if they 
 are not hot enough to allow their lines to be seen as bright 
 lines on the bright background of the continuous spectrum, can 
 still be studied by their absorption, because they are made to 
 vibrate by those wave lengths given off by the interior mass 
 passing through them with which they can synchronise. There¬ 
 fore we pass from those celestial objects which we study by 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 27 
 
 means of their bright lines, to those other bodies which we 
 study by their dark lines ; we pass from radiation spectra to 
 absorption spectra. 
 
 What bodies in the skies then can we get at by this means ? 
 We have already, by means of radiation, been able to gather 
 several secrets from nebulae and from comets, which are the 
 objects which we can study by means of their radiation alone. 
 The most numerous class of bodies in the universe, so far at 
 
 Fig. 11.—Various types of stellar spectra. 
 
 all events as we are able to grasp the universe, are what we 
 term the fixed stars, including, of course, our sun. 
 
 In order to make my meaning quite clear, I will throw 
 upon the screen a drawing which we owe to Father Secchi, 
 which will give you at one view several typical spectra of the 
 fixed stars. When we pass from the nebulae and the comets 
 we pass from bodies which have almost identical spectra 
 in each case to bodies in which the spectra are very different. 
 This diagram shows us the different classes into which the series 
 may be grouped the moment we put this spectroscopic question 
 
28 
 
 MANCHESTER SCIENCE LECTURES. 
 
 to them: What lines have you in your spectrum? or What 
 channelled-spaces have you in your spectrum? We have at 
 the top, you see, the spectrum against which is written the 
 word “ solarand that means that we have there in our sun 
 a representation of a large number of stars, which, be it also 
 remarked, may be large or small, for this classification ap¬ 
 parently does not hold good for the different magnitudes. 
 
 Then, again, we have a spectrum which is common to a great 
 number of the most brilliant stars in the heavens ; and the 
 difference between that spectrum and the upper one is that it 
 has a much smaller number of lines, and that these lines are 
 thicker. Another spectrum is somewhat like the solar spec 
 
 SUN 
 
 POL L U X 
 
 Fig. 12.—The three chief types of spectra seen in more detail. 
 
 trum, so far as the number of lines is concerned, but some of 
 the lines do not agree in position with the lines in the solar 
 spectrum. Now, in the three spectra to which I have already 
 called your attention you see that we have unmistakable line- 
 absorption ; and, in the light of what I ventured to bring 
 before you in- the last lecture, I hope you will quite un¬ 
 derstand that we have evidences in the atmospheres of those 
 stars that the elements are broken down to their ultimate 
 degree of fineness. But when I call your attention to the 
 other four stellar groups, you will find it is no longer a 
 question of line-absorption ; instead, indeed, of a spectrum, 
 resembling the spectrum of calcium and iron, wdiich I showed 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 29 
 
 you in tRe last lecture, we have now most distinct channelled 
 spectra, which will remind you of that beautiful photograph 
 of carbon. That carbon vapour we know was more compli¬ 
 cated than the calcium vapour and iron vapour with which 
 it was mixed. We have then, so far as this diagram can 
 show us, different kinds of absorption going on in the stars ; 
 so much so, that we can divide the stars into groups, first, 
 according to whether or not their absorption is the line- 
 absorption or the channelled-space absorption; and then, 
 again, according to whether the absorption is indicated to us 
 by many thin lines or by few thick ones. I have another 
 diagram here which will enable us to go somewhat more 
 into detail. This diagram we also owe to Father Secchi. It 
 
 Fig. 13. —Spectrum of a Orionis compared with that of Sirius. 
 
 shows you with what extreme care this kind of observation 
 has already been commenced and what detail has already been 
 acquired. We have in the upper part a drawing of the 
 spectrum of a Orionis, from which you will gather that the 
 first drawing which I brought to your notice was only in¬ 
 tended to give you the generic differences between the star 
 classes, and not the special differences. Difference between 
 a star of the type of the sun and of a star of the type of 
 Sirius becomes much more clear and definite when we have 
 the opportunity of observing the enormous number of lines in 
 the stars of the sun type and the comparative freedom from 
 lines of the stars like Sirius and Yega. In order to enter 
 still further into detail in the case of the nearest star, I 
 
30 
 
 MANCHESTER SCIENCE LECTURES. 
 
 will throw on the screen a photograph of a large part of 
 the solar spectrum, which we owe to Mr. Rutherfurd of New 
 York. This will indicate to you the extreme importance 
 of getting the sun to do as much work for us as it can 
 in the way of recording its own chemical constitution by 
 means of photography. This photograph is on such a scale 
 that in order to include a small portion of the spectrum it has 
 been necessary to give it in five successive strips, the less 
 refrangible lines being to the left at the top, and the most 
 refrangible to the right at the bottom. Now, the chemical 
 constitution of the sun and stars, so far as the detail is con¬ 
 cerned, consists in finding out, as I am sure you all know, to 
 the absorption of what particular element each of these lines 
 is due. Now there, for instance, in the line F, we know that 
 we have to do with hydrogen. We know that in the line near 
 
 Fig. 14.—Copy of a photograph of the solar spectrum in the region of the thick calcium 
 lines, by Lockyer. 
 
 G we have to deal with hydrogen again, and that a great many 
 of these very complex lines about G are due some of them to 
 iron, some to calcium, and some to strontium, and so on. 
 Coming to the extreme limit of the visible spectrum, we find 
 lines thicker than all the rest, and those lines we know to be 
 due to calcium. The reason those lines are apparently thicker 
 than all the rest seems to be that probably there is more 
 calcium than anything else in that particular part of the sun’s 
 atmosphere where this absorption takes place. 
 
 Now that remark opens up the kind of inquiry which is 
 possible to us when we wish to inquire into the chemical 
 constitution of the stars. We have the position of the lines, 
 the number of lines, and the thickness of the lines ; and, let 
 me add, when we get definite evidence of change, we want to 
 know the change in the thickness of the lines. Now, when 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 81 
 
 we come to deal with the first class of stars, the brightest 
 and the bluest in the heavens—stars such as Sirius and Yega 
 —much brighter and probably therefore hotter than our own 
 sun—we deal with the extremest simplicity of chemical con¬ 
 stitution. We seem to be dealing almost entirely with hydrogen 
 alone. I say almost entirely, because there appear in the best 
 instruments traces of sodium and magnesium—those metals 
 we have already been familiarised with by our reference to 
 meteorites—in addition to the hydrogen; but that simplicity 
 of construction of the spectrum which you saw on the screen, 
 and the thickness of the lines, taken in connection with the 
 position of those lines, indicates to us that the atmospheres of 
 those stars are composed to very great extent of hydrogen. 
 
 When we pass to the stars of the second class, such as our sun, 
 the chemical complexity is very much greater. If we take the 
 sun as a type of stars of the second class, many of the elements 
 present in its atmosphere have been determined with almost 
 absolute certainty:— 
 
 TABLE OF ELEMENTS IN THE SUN. 
 
 Coronal Atmosphere 
 
 1474 stuff (new element ?) 
 Hydrogen, sub-incandescent. 
 
 Chromosphere . 
 
 ( Hydrogen, incandescent 
 ) Helium (new element ?) 
 I Calcium 
 ( Magnesium. 
 
 Spot Zone 
 
 ! Sodium 
 Titanium 
 Chromium 
 ? Aluminium. 
 
 Reversing Layer 
 
 Iron 
 
 Manganese 
 Cobalt 
 Nickel 
 Copper 
 \ Zinc 
 Potassium 
 Strontium 
 Barium 
 Cadmium 
 , Lead. 
 
 Besides which there are indications that other metals may soon 
 be added to the list, vanadium for instance. The stars then 
 
32 
 
 MANCHESTER SCIENCE LECTURES. 
 
 of the second class, of which our sun is one, have atmospheres 
 composed of these elements. Here, as in the case of the 
 meteorites, our knowledge is already great and is rapidly 
 increasing; but when we come to the red stars, to the stars 
 which give us channelled-space absorption, there up to the 
 present time our knowledge has been extremely limited. We 
 have not been able to study the molecules of elementary bodies 
 and compound bodies under those conditions at which they 
 give us the channelled space spectra to such an extent as we 
 have been able to study them under those conditions at which 
 they give us line-spectra. The result is therefore that when 
 we come to the third class of stars with these channelled spaces, 
 science at the present moment recoils, and is compelled to say 
 that she does not know of what the atmospheres of these stars 
 is composed. But again, in the light of the photograph which 
 I showed you in my last lecture, we can come to certain 
 very definite ideas. For instance, we have no difficulty in 
 coming to the conclusion that the star in which we get the 
 channelled-space absorption must be cooler than the star in 
 which the absorption is of the line kind. It is not at all 
 impossible that science, by taking an entire survey of the 
 whole of space, may, in not a very long period, be in possession 
 of such facts as apparently she could only have acquired by 
 having been present in all points of time ; we may get, in 
 fact, from different regions of space, conditions which have 
 happened to the same body at different epochs of time ; and 
 already it is not, I think, too much to suggest that when we 
 get a star with a channelled space absorption we have got a 
 cooling star, the absorptioi of which must once have been 
 of the line kind ; therefore the stars which now give us line- 
 absorption as they get cooler must give us channelled space 
 spectra, and so on till they become dim and cease to give us 
 light altogether. 
 
 All of you, I am sure, have been struck, one night or 
 another in your lives, with the exquisite colours of some 
 stars. There is no sameness in Nature. The colours of the stars 
 are not so well seen in Manchester or in England generally as 
 they are in the tropics and in more favoured lands ; but still 
 we do, if we take the trouble, easily see evidences of the most 
 beautiful coloration amongst these celestial bodies. Nor is 
 this all. More careful observations of the stars make it 
 absolutely clear to us that they vary very much in the light 
 which they give out; and it is also known that the variation 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 33 
 
 of colour may go on pari passu with the variation of their 
 light. W 7 e owe the first important suggestion on this point 
 to Angstrom, who showed that if in the atmosphere of a star 
 we imagine the molecules at what we may term the critical 
 point, and suppose them to be in a condition of heat which 
 enabled them to give the line spectrum, and also near that 
 reduced temperature and possible association at which they 
 could give the channelled space absorption, a very small reduc¬ 
 tion of temperature would at once change utterly the amount 
 of light given out by that star. For instance, if you assume 
 that stars of the third class were once stars of the second, we 
 know that if the change could have taken place suddenly, it 
 would have appeared as if these stars lost a great deal of their 
 light, and that the yellow light was gradually changed to 
 red. Now the variability of stars can go to such an extent, as 
 I have already hinted, that a star will go out altogether, so far 
 as our powers of seeing it are concerned. And again we may 
 perceive a star in a region of the heavens where a night before 
 no brilliancy was visible. How can we account for this 1 So 
 far as the physics of the stars are concerned, the merely chemi¬ 
 cal considerations would at once explain to you how it was 
 that a star should by and by lose its light. The cooling of the 
 atmosphere, and the consequent increased absorption of the 
 molecules of that atmosphere, as they got more complicated 
 by the reduction of temperature, would be quite sufficient to 
 stop all light which came from the nucleus. When we inquire 
 how it comes that a star may suddenly shine out where no 
 star ever shone before, there the law of uniformity, the law of 
 continuity, does not come to our rescue so well as it does in 
 the first case. We do want there something like a catastrophe. 
 You recollect that a few years ago Dr. Huggins was enabled to 
 make a most important series of observations on a star which 
 broke suddenly into intense light and then faded away into 
 the utmost bounds of visibility. Now there it was found that 
 the light of the star, changing as it did, gave rise to perfectly 
 different spectrum effects. That star when it was only of a 
 certain definite brightness, as it was at first seen, gave us a 
 spectrum with dark lines similar to those I have thrown on 
 the screen; but when its maximum brilliancy was reached, 
 the character of the spectrum changed—a spectrum of bright 
 lines was added ; and then we had an indication of a new class 
 of bodies, namely, those bodies which we study both by their 
 radiation and absorption. So much for the facts. 
 
34 
 
 MANCHESTER SCIENCE LECTURES. 
 
 The Sun. 
 
 We will now pass to the sun, so that we may be able to 
 build any conclusions with regard to the causes of these 
 phenomena in the case of the more distant bodies on some¬ 
 what firmer foundations than we could have done had we 
 not this big star close to us to refer to. In the first place, 
 I would like to show you that the statements which have 
 been made with regard to the chemical constitution of the 
 stars rest upon a basis sufficiently firm to justify me in bring¬ 
 ing them before you. Dr. Huggins, who was the first to 
 observe the spectra of the stars in a manner which left 
 nothing to be desired, so far as eye observations were con¬ 
 cerned, made comparisons of the dark lines of the stars with 
 the bright lines of the different elements in the same instru¬ 
 ment at the same moment. A very small addition to that 
 method, namely, the introduction of photography, enables us 
 not only to do this, but to make a record which is good for all 
 time. I propose therefore to illustrate the method by throw¬ 
 ing upon the screen two photographic comparisons, the object of 
 which was to determine which were the lines I have already 
 shown you in the solar spectrum, which were really due 
 to the vibration of the particles of iron vapour in the atmo¬ 
 sphere of the sun. For that purpose all that one had to 
 do was first to photograph the spectrum of the sun, and 
 then on the same plate and by the same instrument, under 
 absolutely the same conditions, photograph the spectrum 
 given by the iron vapour. You will see the result on the 
 screen. Of course such a photograph has to be made for every 
 chemical element the existence of which in the sun we wish 
 to study. I may remark, in passing, that the only difficulty 
 in illustrating this kind of inquiry is the impossibility one 
 labours under of ever getting a chemical substance which is 
 absolutely pure. We have now on the screen, on a very large 
 scale indeed, that part of the solar spectrum which in Mr. 
 Hutherfurd’s photograph was to the extreme right at bottom. 
 Here are the two calcium lines which you saw before, and 
 which are much thicker than any other lines in the spectrum. 
 The dark lines are the regions where there is no light to paint 
 the image of the slit in consequence of that light having been 
 absorbed by the iron vapour in the atmosphere of the sun ; and 
 above these dark lines you have the images of the slit painted by 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 35 
 
36 
 
 MANCHESTER SCIENCE LECTURES. 
 
 the vibrations of iron vapour, not in the sun’s atmosphere, but 
 in a laboratory at South Kensington. If you will take the 
 trouble to compare the more definite lines you will see that 
 there is a perfect coincidence between the bright lines and the 
 dark ones which are caused by the iron vapour of the sun. 
 
 The next diagram 1 is, if anything, more interesting still. In 
 this one we are dealing not with iron but with manganese. 
 We have, you see, bright lines coincident with the dark lines 
 of calcium, but these are due to calcium impurities. I am 
 anxious to call your attention to a group of four lines in the 
 solar spectrum and a broad band of light, corresponding with 
 these in the spectrum of manganese. There are three bands 
 of absorption on this band exactly coinciding with the three 
 more refrangible lines in the solar spectrum to which I have 
 drawn attention ; that is to say, we not only in that photo¬ 
 graph get absolute proof that those four lines in the solar spec¬ 
 trum are due to absorption at the sun by vapour of manganese, 
 but we get the vapour of manganese in a laboratory doing for 
 the more incandescent manganese what the outside sun does for 
 the inside sun ; we have in fact the cool vapour of manganese 
 around the incandescent manganese giving us nearly the same 
 absorptive effects as the manganese vapour does at the sun. 
 
 So much then for the method of acquiring these chemical facts. 
 If we merely had that method, we should be able to say that 
 certain substances exist in the sun; but that would scarcely be 
 enough. We don’t want to know merely that such and such sub¬ 
 stances exist in the sun or in a distant star ; if possible, we want 
 to know whereabouts in that star the particular substance lies. 
 Now for that purpose we have to consider these spectroscopic 
 results in connection with the telescopic results. What I mean 
 by telescopic results will be brought before you by throwing 
 on the screen in the first place a photograph of the sun 
 which again I owe to the kindness of Mr. Kutherfurd of 
 New York. Here is the sun on an enormous scale, photo¬ 
 graphed by itself. 
 
 The Chemistry of Different Portions of the Sun. 
 
 Now if instead of observing the sun as ordinarily visible, we 
 observe it during an eclipse, we find that the sun that we see 
 is only the small interior nucleus, so to speak, of the true sun, 
 
 1 This photograph is not given, but the same effect may he noticed in 
 Fig. 1 in the case of the thicker lines of calcium and aluminium. 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 37 
 
 and the reason we see the interior nucleus alone is because it 
 is so very much more bright than the surrounding atmosphere. 
 This photograph (Eig. 16) was taken in India in 1871. When 
 we get the dark moon exactly between us and the brighter 
 interior sun, there is no difficulty whatever in seeing that 
 there is an atmosphere with its own characteristic effects, 
 extending to a considerable distance above what we consider 
 the sun ordinarily speaking (Fig. 17). So that you see 
 telescopically we can make a complete distinction between one 
 part of the sun and the other. 
 
 Now the question is, can we spectroscopically determine in 
 what particular part of the sun each of these elements exists 1 
 
 Fig. 16.—The Sun’s corona, from a photograph taken in 1871. 
 
 Here is another drawing, which shows you what happens when 
 we observe the region intermediate between the two I have 
 shown you. The first drawing brought before you the 
 photosphere of the sun ; the second drawing brought before 
 " you the corona—the name given to all the exterior of the sun. 
 At the base of the coronal atmosphere, that is, just above the 
 photosphere, we have a region which has been named the 
 chromosphere, in which certain strange forms are to be seen, 
 and which are here shown (Fig. 18) from a drawing by 
 Professor Young. These have been called “ prominences/’ or 
 “ red flames.” 
 
 ►So that we have the photosphere underlying part of the sun’s. 
 
38 
 
 MANCHESTER SCIENCE LECTURES. 
 
 atmosphere and above all the coronal atmosphere ; resting in 
 the middle, so to speak, this chromosphere and its prominences ; 
 and then between the photosphere and the base of the chromo- 
 
 Fig. 17.—The Sun's corona and prominences, sketched during the eclipse of 1S68. 
 
 sphere an extremely thin layer where most of the absorption 
 takes place. Now this thin layer has been called the reversing 
 layer, because it is here that the sun’s light is reversed and 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 39 
 
 the Fraunhofer lines produced ; so that the statements generally 
 made as to the chemical constitution of the sun and stars really 
 refer to this particular film—for film it is, in comparison to the 
 si2e of the sun—lying between the chromosphere and photo¬ 
 sphere. We have, therefore, the photosphere, reversing layer, 
 chromosphere and coronal atmosphere, into which, if we can, we 
 may sort out the different elements. 
 
 Now this is what has been done, and the results are shown 
 in the table (page 31). 
 
 If you imagine an arrow shot into the sun, it would first 
 pass through the unknown element of which the upper part 
 
 Fig. 18.— Young’s drawings oi prominences. 
 
 of the coronal atmosphere seems to be chiefly composed; it 
 would then come to the region where practically there is 
 hydrogen and nothing else; getting into the chromosphere you 
 come to another new element, and then in the reversing layer 
 you get magnesium, calcium, sodium, &c. At length we come 
 to traces of some substances which either exist there in very 
 small quantities or lie at a much lower level than the others. 
 
 For one of the important teachings of this work seems to be 
 that at the great temperature of the sun—a temperature 
 which brings about a dissociation much more complete than 
 
40 
 
 MANCHESTER SCIENCE LECTURES. 
 
 anything we can obtain here even by electricity, unless perhaps 
 we use the most powerful induction coils—there is the same 
 magnificent order, though it may not be fully known, which 
 exists throughout nature, and we find hydrogen thinning out 
 at one level and magnesium thinning out at another, and so 
 on ; and I have suggested that the system of strata produced 
 by this thinning out may be connected with the true vapour 
 densities of the elementary bodies. 
 
 With regard to magnesium and calcium, I should remark 
 that the list, made in 1874 will most probably have to be 
 revised, as one of the results of the Eclipse Expedition to 
 Siam in 1875 ; 1 for although in all past work on the sun it 
 was impossible to determine whether magnesium or calcium 
 was highest, the Siam observations seem to add to the pro¬ 
 bability that calcium really lies above magnesium ; in other 
 words, that the true vapour density of calcium is less than 
 the true vapour density of magnesium. This reference to 
 vapour densities will in the next lecture bring me into com¬ 
 plete rapport with Professor Boscoe’s lectures on the chemical 
 structure of the earth’s crust. 
 
 The Planets. 
 
 There is still, however, another large group of bodies to be 
 considered—I refer to the Planets—before we have exhausted 
 the bodies external to the Earth. Although, however, this 
 group of bodies is numerous, we have not very much to say 
 about them, for a reason which I am sure you will easily 
 appreciate. When we deal with the radiation of the 
 heavenly bodies, we are dealing with a condition of vibra¬ 
 tion of particles of those bodies at their utmost fineness, 
 so that each vibration comes to us with a story to tell as 
 to the actual chemical constitution of that body. Then when 
 we come to that large class of objects which we study by 
 means of absorption, there again we have the same mole¬ 
 cules doing the same thing, but instead of giving us vibra¬ 
 tions of their own, they absorb other vibrations which were 
 attempting to pass through them. But when we come to 
 the planets, we come to bodies like our own earth ; bodies 
 comparatively cool ; bodies not in a state of incandescence, 
 
 where matter is not as a rule in a state of gas or vapour, but 
 
 • 
 
 1 I have done this in the table as given. 
 
WHY THE EARTH'S CHEMISTRY IS AS IT IS. 41 
 
 in the solid form. We come therefore to bodies which can 
 neither radiate nor absorb light, in the sense in which we 
 have dealt with radiation and absorption; because, in conse¬ 
 quence of the reduction of their temperatures the chemical 
 elements have compounded ; we have not the individuality 
 which is requisite; we have not the discrete particles, but 
 combinations of every complexity. As the result of that, 
 what is our only chance of seeing them at alH We see them 
 by reflected light. The bodies now under consideration, un¬ 
 like thfe nebulae and comets, and unlike the stars, reflect light 
 to us, and only by the reflection from their surfaces can we 
 tell that they exist. Now as all bodies, whether they are solid 
 or liquid, are spectroscopically dead,. so to speak, so far as 
 getting chemical information from them is concerned, inquiry 
 is perfectly useless excepting in one particular—it proves that 
 it is powerless, by showing that the light of the sun is so faith¬ 
 fully reflected by these bodies, that all the principal lines of 
 the solar spectrum are to be found in it. It is true that there 
 are exceptions in the case of the exterior planets of our system, 
 especially in Uranus and Neptune. In the spectrum of those 
 bodies, cool though they are, like our own, in addition to a 
 constant absorption of the sun’s atmosphere and the earth s 
 atmosphere, a third absorption of the atmosphere of the indi¬ 
 vidual planet is indicated With that exception, you will see 
 that the spectroscope is powerless to help us. How then can 
 we hope to get at the chemical constitution of the planets if the 
 spectroscope does not come to our assistance ? There is, I fear, 
 only one chance for us, and that is to determine, as nearly as may 
 be, the densities of these bodies, and to see if it is possible to find 
 out anything to reason upon when these densities have been 
 thoroughly well sifted. Now we do know already with some 
 accuracy the density of the planets. We know that these 
 planets may be broadly divided into two groups ; we have the 
 interior group, of which the Earth is one—Mercury, Yenus, 
 the Earth, Mars—small heavy planets. The density of the 
 earth is about five and a half times the density of water. The 
 density of these interior planets you may say, roughly, is the 
 same as the density of the earth ; therefore we have this 
 group of interior planets with a density of five and a half 
 times that of water. After this we have a considerable gap, 
 a gap partly filled by the minor planets or asteroids ; and after 
 that we get another group—Jupiter, Saturn, Uranus, Neptune 
 not small and dense planets like the Earth, but enormous 
 
42 
 
 MANCHESTER SCIENCE LECTURES. 
 
 light planets, having, roughly speaking, and on the average, 
 about the density of water. So that the density of the interior 
 planets is to the density of the exterior planets about as five 
 to one. NTow if this density were known to be associated, in 
 the case of the planets I have named with equal solidity from 
 centre to circumference, of course we should be able then to 
 form a rough idea as to their composition. But we do not 
 know that. But still let us, if we can, carry the inquiry into 
 the secondary bodies of this system—into their satellites. 
 
 If we take the only case in which facts approximately accu¬ 
 rate are at our disposal—the case of Jupiter—we find that if 
 we take the density of the satellites of Jupiter to be on an 
 average one, the density of Jupiter is five, and the density of 
 the Earth as an interior planet would be twenty-five ; so that 
 the outside planets of our system are one-fifth the density of 
 the inside planets ; and in the only case where we have a 
 complicated system of satellites, that we can deal with, we 
 have exactly the same relationship there, and the satellite is 
 only one-fifth of the density of the planet itself. 
 
 I hope to have the opportunity next week of pointing 
 these remarks by reference to what I have already brought 
 before you, especially to the condensation of the nebulae and 
 to the particular position which each chemical element occupies 
 at the present moment in the atmosphere of the sun. 
 
LECTURE III. 
 
 [n the latter part of the last lecture I referred to the different 
 densities of the two great planetary groups. We saw the in¬ 
 terior group of a density, roughly speaking, five times greater 
 than that of the exterior group ; and taking the satellites of 
 one body in the exterior group, we found the same relationship 
 of density ; the primary being five times as dense as the 
 secondary body, which in that case was one of the satellites of 
 J upiter. 
 
 Now the fact that the Earth is one of the interior group of 
 planets leads us to assume (and 1 pointed out to you that 
 assumption is almost the only thing left to us in regard to 
 estimating the chemical relationships of the earth) that 
 probably the chemical constitution of the earth is similar to 
 that of the planets which form the interior group—Mercury, 
 Venus, the Earth, and Mars. Now if we look upon the planets 
 from another point of view, if we consider the extent to which 
 some of them are flattened at the poles, we find the same 
 grouping as we did before. The interior planets are 
 flattened very little at the poles, as compared with th 
 flattening of the exterior bodies. Now this flattening has 
 been very beautifully experimented upon by Professor Plateau: 
 and, thanks to Mr. Binyon’s skill, I hope I shall be able 
 to throw on the screen some of the phenomena to which Pro¬ 
 fessor Plateau refers. When it is a question of investigating 
 the flattening of a planet experimentally, the first thing one has 
 to do is to take away the influence that gravity might have on 
 the body experimented upon; and Professor Plateau very 
 ingeniously did this by making the rotating body a, mass of 
 oil in a mixture of spirit and water of precisely the same 
 specific gravity ; so that the mass of oil in the centre was 
 neither inclined to rise nor fall, if the mixture had been 
 
44 
 
 MANCHESTER SCIENCE LECTURES. 
 
 properly made. Here we have on the screen an image of such 
 a mass of oil and a disc connected with a spindle, which we can 
 cause to revolve somewhat rapidly. The revolution of the 
 spindle is communicated to the oil by means of the disc, and 
 what we find is this (supposing the experiment to be perfect). 
 With a certain amount of rotation, the spherical form of the 
 oil first changes into a spheroidal one; as the rotation is in¬ 
 creased we get a flattening—as the mass of oil is compressed 
 in one direction it is extended in the other—and we get the 
 equivalent of what we have in the Earth which we describe 
 by saying that the equatorial diameter is so much greater 
 than the polar diameter. When we are able to repeat this 
 beautiful experiment under the best conditions, we find that 
 after a certain point, the oil is not content with expanding 
 in one plane; it is not a question of shortening one diameter 
 and increasing another, but under one set of conditions the 
 oil can be made to form a complete ring, absolutely perfect 
 and disconnected from the central disc ; and when the rota¬ 
 tion of the central disc is slackened, the oil then comes back 
 again and re-forms, so to speak, a miniature planet. That 
 is one case. Another case can be studied by commencing the 
 rotation with somewhat greater rapidity; and what happens 
 then is that, instead of getting the formation of a ring, the 
 oil is broken up and thrown off in tangents, forming a kind 
 of spiral. Those are the two main classes of phenomena 
 which can be observed in this way. 
 
 The interior group of planets has a day almost entirely 
 the same as ours—a period of rotation of about twenty-four 
 hours. The period of rotation of the exterior planets has 
 not been determined in the case of the two exterior ones, 
 Neptune and Uranus ; but we do know that in the case of 
 Jupiter and Saturn the rotation is accomplished in less than 
 half the time taken by the members of the interior group. 
 
 What, then, are the facts with regard to these planets and 
 their flattening ? I am able, by the kindness of several friends, 
 to throw upon the screen some very beautiful drawings of 
 all the planets which I have mentioned; and I want you to 
 be good enough to look at these drawings from two or three 
 points of view. First, I want you to see the difference in the 
 amount of the polar compression in each case ; and, for future 
 reference, also the difference in the atmospheric effects. We 
 will begin then with the planet which is most similar to our 
 own, the planet Mars. 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 45 
 You will see that it has markings similar in kind, no doubt, 
 
 Fig. 19. —Mars ; south pole visible. 
 
 Fig. 20.—Mars; both polar caps visible. 
 
 to the markings which would be seen by the spectator observ- 
 
46 
 
 MANCHESTER SCIENCE LECTURES. 
 
 ing the Earth from the Moon. You will see also that its 
 compression is small—in fact, I may say, that it is not to he 
 appreciated at all. Here are three drawings of Mars, made by 
 the distinguished Hutch astronomer, Kaiser, of Leyden. You 
 see that there is no polar flattening. That the upper part 
 represents the true pole of the planet is rendered evident by 
 the fact that you have there a snow cap at the south pole. 
 There again you have the snow cap ; and here in these dark 
 markings we have seas. The Earth’s place then, in Nature, 
 both as to polar compression and atmospheric condition, is 
 evidently very similar to that of Mars. When, however, we go 
 
 Fig. 21.—Jupiter. 
 
 from Mars, which is the only member of the interior group, 
 excepting the Earth, about which we can say anything with 
 decision, we see that all the phenomena are considerably 
 changed. We pass from a density of five to a density of one, 
 and the twenty-four hours day or thereabouts of Mars is now 
 replaced by a day of something like ten hours in the case of 
 Jupiter, the planet which comes next in our survey. Here we 
 see how much shorter the polar diameter is than the equa¬ 
 torial one. You will be reminded by these cloud-belts of the 
 much more simple system of cloud-belts which traverses our 
 own Earth near the equatorial regions. There is little 
 
WHY THE EARTH'S CHEMISTRY IS AS IT IS. 
 
 47 
 
 doubt that the darker portions here are the portions of the 
 atmosphere of the planet freest from cloud, and it is espe¬ 
 cially in this region that an observation to which I shall 
 presently have to refer was made. Going then still outwards, 
 we come from a compression of considerable magnitude to a 
 planet in which the compression is somewhat less. But you 
 will see, that although the polar compression is somewhat 
 less, we have what I termed an “ extreme case,” when I was 
 referring to Plateau’s experiment. We have in the planet 
 before you (Saturn) exactly the condition which was observed 
 by Plateau in his experiments with the oil and mixture of spirit 
 and water. We have traces of clouds, as in Jupiter; but the 
 all-absorbing feature in the case of Saturn is this wonderful 
 ring, about which observations are, fortunately for science, being 
 very rapidly accumulated, showing that considerable changes 
 are going on in it. 
 
 We now know that we are in presence of a ring, or rather 
 an infinite series of rings, of, let us say, meteorites, small 
 satellites of Saturn, out of which at some future time larger 
 satellites will be compounded. This is one of the most beautiful 
 results of modern thought and work. Laplace, who first con¬ 
 sidered the question of the mechanics of the rings, which were 
 in his time considered to be solid, was content to leave them 
 solid, provided the rings were very numerous and that the centre 
 of gravity of each was not coincident with the centre of gravity 
 of the ball; but modern mathematicians, among whom must 
 be specially mentioned Peirce and Clerk Maxwell, have shown 
 that the rings cannot be solid and cannot be liquid, and in 
 short such a structure as that referred to above is the one now 
 required by mathematical theory. Such a structure, more¬ 
 over, is the only one which fits the facts. The brightness of 
 different portions, the variations in brightness and breadth of 
 each bright or dark part, the gradual widening of the whole 
 system—29 miles a year according to one estimate—and many 
 other facts are thus easily explained. Some recent observations 1 
 made by the Washington 26-inch equatorial not only establish 
 important changes which have recently been going on, but afford 
 further evidence of the meteoric structure of the strange append¬ 
 ages, £.</., the dusky inner ring is not now perfectly transparent as 
 it once was, the planet can only be partly seen through it, while 
 the matter composing it is agglomerated here and there into 
 small masses, which prevent the planet being seen. Mottled 
 1 Trouvelot, Proc. Amer. Acad. 1876, p. 174. 
 
Fig. 22.—Saturn, from a drawing by Trouvelut, made by the Washington 2b-inoh refractor. 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 49 
 
 or cloudy appearances have also been observed on the surface 
 of the exterior portions of the ring system during the last four 
 years. 
 
 We must now pass from the facts relating to the physical 
 condition of the two great groups, as it is as yet impossible by 
 a further discussion of them to learn anything about the 
 chemical constitution of the planets themselves. The question 
 arises—Can we learn anything about the composition of their 
 atmospheres ? Let me remind you that we are dealing with 
 that class of bodies which shine by reflected light. It is clear 
 therefore that when we examine by the spectroscope the light 
 of the sun reflected by these bodies, we shall have the solar 
 spectrum, plus the spectrum due to the absorption of any special 
 planet. How, as a matter of fact, the solar spectrum, as ob¬ 
 served from the Earth, is tainted by, or mixed up with, the 
 absorption of our own atmosphere. But fortunately we can 
 get rid of the absorption effect of our atmosphere by varying 
 the observations so that at one time we shall have a great 
 thickness of atmosphere, as when we observe the sun in the 
 morning or evening, and at other times a small thickness, as 
 when we observe at mid-day ; and at those times we shall 
 have the spectrum changed, owing to this change of condition. 
 In that way men of science have been able to separate the 
 absorption taking place at the sun, from the absorption due 
 to the Earth’s atmosphere. 
 
 The interior planets tell us that there is absolutely no 
 special absorption in their atmospheres. So far as they have 
 atmospheres at all, they are undoubtedly similar to our own; 
 therefore the Earth’s place in Nature is with the interior 
 groups of planets. But when we pass outwards from the 
 interior group to the uttermost confines of the exterior one, 
 when we leave Mars to go to Neptune, Saturn, and Uranus, 
 we find that from Jupiter, outwards, there is a something 
 interpolated into the atmosphere, so that the outermost planet 
 has the atmosphere which differs most from our own. Uranus 
 and Neptune have very extraordinary atmospheres of their 
 own, which are indicated by a very definite spectrum. Traces 
 of the substance which gives us this extraordinary absorption 
 in the outermost planets are also to be found in the atmospheres 
 of Jupiter and Saturn ; so that we are driven to the conclusion 
 that the atmosphere of the exterior planets is different from 
 the atmosphere of the Earth by the addition of a new absorb¬ 
 ing substance to the aqueous vapour which is the only effective 
 absorber in our own atmosphere. 
 
50 
 
 MANCHESTER SCIENCE LECTURES. 
 
 The Question of Evolution. 
 
 I have now gone through, seriatim, the physical and 
 chemical constitution of nebulae, stars and planets, so far as 
 the facts and the time at my disposal have enabled me. What 
 may we say then as a general summation of these facts, 
 brought together with a view of determining whether they 
 throw any light on the cause of the Earth’s actual chemical 
 constitution ? Physically speaking, the Earth is a cooled body, 
 revolving round an incandescent one. Chemically speaking, its 
 chemistry has been most admirably brought before you by Dr. 
 Roscoe in those three lectures which we have now the oppor¬ 
 tunity of reading carefully; and its kinship with the other 
 bodies which people space has been established by the fact of 
 the community of elements. Dealing only with the spectra 
 which have been observed in the nebulse we have recognized 
 hydrogen ; in the comets we have recognized hydrocarbons ; in 
 the stars and in the sun we can tell of elements amounting 
 to twenty-five in the case of the sun, nearly all of which 
 elements we have on this Earth; so that it is not too much to 
 say that one of the glories of the spectroscope has been to show 
 that the Earth in its chemical constitution is simply a pai*t of 
 one of the great systems of the universe, and differs in no way 
 from the matter which masses here as nebulse, there as comets, 
 and there as stars. 
 
 I have drawn your attention to the conclusions which we are 
 justified in drawing from the facts; and the question is, Does 
 the story end now the facts end ? I hope to show you that it 
 need not necessarily do so. A student of science endeavours 
 to accumulate facts. Facts are the rewards of scientific work ; 
 but how, when a man of science has accumulated a large 
 number of facts, is he best to proceed to get new ones h That 
 is our case. I have endeavoured to bring before you all the 
 facts, roughly, which the spectroscope has placed at our dis¬ 
 posal with regard to the chemical constitution of those 
 different masses of matter which people space. But how are 
 we to attempt to get more ? Are we to go haphazard at it ? 
 NTo. Hypothesis is the life-blood of investigation; and having 
 an array of facts before you, the thing to do is to attempt 
 to put them in such an order that they will suggest inquiry 
 in -special directions. Do not be afraid when I say that hypo¬ 
 thesis is the life-blood of investigation, lest the world should 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 51 
 
 soon be filled with hypotheses. History does not bear this out; 
 and further, a man does just as good work by destroying an 
 unsound hypothesis as by establishing firmly a true one. 
 Having then these facts before us, and having necessarily to 
 form some hypotheses, what is the most general question that 
 we can put to these facts, which tell us of the existence of 
 nebulae, comets, suns, planets, and the like h The most general 
 question, I think, that we can ask is such a one as this : Have 
 we, as the result of our inquiry, got together such facts as 
 enable us to consider nebulae, stars, comets, and planets as 
 finished products, each of them gloriously, magnificently, perfect 
 in its way ; or do these merely represent, let us say, the seed 
 and the flower and the fruit ] 
 
 Here, of course, we have to do with Evolution pure and 
 simple; and I may remind you that it is now a good many 
 } r ears since two of the most profound thinkers on our planet 
 at the time—I refer to the German and Frenchman, Kant and 
 Laplace—independently arrived at the answer to the question 
 I have propounded. The answer was identical, and it was 
 this : You have in these various bodies not finished products, 
 each perfect after its kind, but really and truly the seed 
 and the flower and the fruit. And the hypothesis which they 
 put forward was something like this—that the nebulse, repre¬ 
 senting what I told you Tycho Brahe considered them to be— 
 a true fire-dust—was probably wrought into stars by means of 
 the condensation due to gravity, which in time brought the most 
 irregular nebulse down to the appearance and the consistence 
 of a star. And then Laplace and Kant, knowing well of the 
 observations of the rings of Saturn—the experimental imitation 
 of which I have brought before you—suggested that our planets 
 were left behind, first as rings, while the nebula was contract¬ 
 ing to form a star. So that if you begin, in any quarter of 
 space that you choose, with a nebula, and give it time to work, 
 the nebula will condense, a star will be formed, and in the pro¬ 
 cess rings, which will subsequently break up and form planets 
 which will subsequently cool, will be produced. Such then is 
 an hypothesis which has now been before the world some years 
 and certainly is exceeded by none in the grandeur and grasp 
 of its conception ; it was started before the spectroscope, as we 
 now know it, was dreamt of. Do the facts which have been 
 brought to light by the spectroscope lead us to think that 
 this hypothesis will no longer hold water now we have got 
 a larger area of facts to deal with, or do the new facts really 
 
52 
 
 MANCHESTER SCIENCE LECTURES. 
 
 come well in and support the old and enable us to fit other 
 parts into it *1 
 
 NTow, in order to deal with this question, I shall first trace 
 the passage from the nebula to the star, which is the first part 
 of the conception of Kant and Laplace. If you will consider 
 the conditions at work in a nebula such as I told you 
 Professor Tait is content to imagine it, namely, a nebula 
 consisting of a huge cloud of stones, the luminosity of which 
 depends chiefly upon the impact of these stones together, and 
 I suggest in part upon dissociation of hydrocarbon—if you 
 take such a nebula as this, and imagine it to be condensed 
 to a star, you will see at once that the conditions of Plateau’s 
 famous experiment are exactly reversed. In the experiment, 
 we begin with the star at rest, and the experiment is per¬ 
 formed by setting it in motion. But in truth, if Kant was 
 right, that experiment must be exactly reversed ; rotation will 
 be at the end of the business. We must imagine rotation set 
 up over an immense area, rapid enough, after the condensation 
 in a plane is considerable, to give us many rings. What 
 Plateau saw when the matter went outwards will occur back¬ 
 wards, as the matter condenses inwards. Instead of rings 
 formed and moving outwards we shall have rings formed and 
 moving inwards, tangential outpourings will be represented by 
 tangential inpourings. I will recall your attention to two or 
 three nebulse, which I showed you on a former occasion, and in 
 the light of this hypothesis, I think you will see that the 
 telescopic evidence is also in favour of the idea. I am now 
 about to show you the group of nebulae sketched by Lord 
 Posse, some of them having been observed in the first instance 
 by Sir John Herschel. I do not wish to call your attention 
 to all of them, although much might be said, I am sure, about 
 every one ; but I want especially to draw your attention to 
 those which res:mble Saturn, and are like the sphere and 
 ring of oil in Plateau’s experiment. We have in the centre 
 a condensation; we have around it a globular mass not so 
 brilliantly illuminated; and around it again we have a ring. 
 There again is an object of an almost similar kind, seen 
 from a different point of view, in which the ring is edge¬ 
 ways to us. I might go on taking up very much more of 
 your time, but I don’t think it is necessary to enlarge very 
 much upon this part of the subject, after the drawings I 
 showed you on the first occasion. It will be clear to you that 
 if we take this view we shall demand with each degree of 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 53 
 
 condensation, with each degree of contraction, an increase 
 in brilliancy; so that the smaller the space occupied by the 
 materials which first constituted the nebula, the brighter 
 and the hotter will they become. So that we must imagine 
 that the star finally formed must be one in an intense state of 
 incandescence ; in fact, in a state of incandescence of which we 
 cannot have the most remote notion. 
 
 What says the spectroscope % Here the spectroscope is quite 
 at one, I think, with the idea; in fact, there are nebulae and 
 stars with spectra so similar that if one had the evidence of 
 the spectroscope alone, it might be impossible to decide which 
 was nebula and which was star. Now this may be a little 
 startling to some of you, and therefore it is only fair I should 
 explain it. The stars, you know, are so remote from us that 
 in the most powerful telescopes to which spectroscopes are 
 applied, they appear only as the finest points of light. Now 
 these points of light, it is not absurd to imagine, may in some 
 instances be two millions, or perhaps even three millions, of 
 miles in real diameter. We know that our own sun, which is 
 certainly not the largest star in the heavens, is nearly one 
 million miles in diameter; that is to say, the true sun, the true 
 stellar nucleus, is one million miles in diameter. Now when I 
 dealt in my second lecture with the physical constitution of the 
 sun, I pointed out that the sun which we see, the sun which 
 sends us the majority of the light we receive, is but a small 
 kernel in a gigantic nut, so that the diameter of the real sun 
 may be, say, two million miles. Suppose then that some 
 stars have very large coronal atmospheres; if the area of 
 the coronal atmosphere is small compared with the area of 
 the section of the true disc of the sun, of course we shall get 
 an ordinary spectrum of the star; that is to say, we shall get 
 the indications of absorption which make us class the stars 
 apart; we shall get a continuous spectrum barred by dark 
 lines. But suppose that the area of the coronal atmosphere is 
 something very considerable indeed, let us assume that it has 
 an area, say fifty times greater than the section of the corona 
 of the star itself; now, although each unit of surface of that 
 coronal atmosphere may be much less luminous than an equal 
 unit of surface of the true star at the centre, yet if the area 
 be very large, the spectroscopic writing of that large area will 
 become visible side by side with the dark lines due to the 
 brilliant region in the centre where we can study absorption ; 
 other lines (bright ones) proceeding from the exterior portion 
 
54 
 
 MANCHESTER SCIENCE LECTURES. 
 
 of that star will be visible in the spectrum of the apparent 
 point we call a star. Now it is difficult to say whether such a 
 body as that is a star or a nebula. We may look upon it as 
 a nebula in a certain stage of condensation; we may look upon 
 it as a star at a certain stage of growth. 
 
 And in the fact that we have actual nebulous stars—stars 
 which in the telescope can scarcely be defined from true stars, 
 and spectroscopic effects such that it is difficult for us to say 
 whether they are produced by star or nebula, we have in these 
 two points very strong arguments indeed in favour of this part 
 at all events of the evolution hypothesis. 
 
 So much then for the passage from nebula to star. 
 How about the passage from star to planet ? Let us, in the 
 first instance, assume that the nebula is vastly condensed 
 before the rings begin to form. It is difficult of course for us 
 to say whether this is really true or not, because we know not 
 the distance of the nebulse ; but I think you will agree that it 
 is a fair assumption. In this case, if the condensation is ex¬ 
 cessive, and the heat due to arrested motion be excessive also, 
 we shall have then to deal with the facts. Where in the heavens 
 have we just this particular series of facts f l We have it in 
 the sun. We have the sun now near to us, so that we can study 
 its constitution, still in a state of intense incandescence, and 
 so far as that point goes at all events it may be fairly taken to 
 represent probably an earlier stage of its growth. And this 
 being so, I think it will be fair to take the sun as repre¬ 
 senting the closest approximation to a nebula in its last stage 
 which is available to us. It would be better perhaps if the 
 line between the inner and outer atmospheres were less sharp, 
 but we must take it as it is. Now one of the most unforeseen 
 results connected with recent solar work has been that the 
 different chemical substances which various observers have 
 placed in the atmosphere of the sun, are not all mixed up pell- 
 mell, but are really thinned out into layers ; so that we can not 
 only say, when we wish to compare the chemical constitution 
 of the Earth with the sun, that the chemical constitution 
 of the sun is so and so, but we can say that in the chemical 
 constitution of the sun this element occupies one position in 
 the solar atmosphere, and that element occupies another. So 
 that, in fact, taking the sun to represent a nebula at the time 
 that rings were being formed (mind you, I don't say thrown 
 off, we must reverse Plateau’s idea), the fundamental point we 
 have to bear in mind is that if we take the sun at all, we must 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 55 
 
 take it with its layers. Now here we have an opportunity 
 which Kant had not; and here, I think, the spectroscope 
 enables us to deal with Kant’s hypothesis in a somewhat satis¬ 
 factory manner. We may be said to improve somewhat upon 
 the conceptions which were merely physical by making them 
 slightly chemical. Now if we have these known metals placed as 
 near as may be in this order [Table]—probably the position 
 of the metal aluminium is the most doubtful,—we have in 
 the first instance to account for the absence of the metalloids. 
 I have given grounds elsewhere, and it is not necessary to 
 repeat them to-night, for believing that the most probable 
 position for the various metalloids in the solar economy is out¬ 
 side these metallic strata. So that if we take a section of the 
 sun’s atmosphere, we must imagine the metalloids to be outside 
 if they be there at all, and then as a link between the metal¬ 
 loids and denser metals, hydrogen, calcium, and magnesium 
 generally coming in that order, or thereabouts. Let us for a 
 moment consider the sun as a nebula. There is ample evi¬ 
 dence, I think, to show that the temperature of the nebula 
 was then as great as the temperature of the sun is now ; 
 consequently you will have all these metals existing uncom¬ 
 bined, the metalloids existing outside, also probably uncom¬ 
 bined. And what happens on Kant’s hypothesis?—that the 
 nebula in starting a rotation and contracting leaves behind it its 
 exterior portion ; so that probably we may say that the first 
 planet thrown off—let us assume that that was Neptune, 
 although probably there are many planets -beyond Neptune— 
 must have been thrown off from the extreme limit of such a 
 nebula, and must have been built up of those particular mate¬ 
 rials which were existing at that particular part of the nebula ; 
 that is to say , that it is almost impossible that there should not be 
 an overwhelming preponderance of metalloid in Neptune. As the 
 various rings are formed from the exterior of the n'ebula while 
 the contraction is going on, they will consist chiefly of metal¬ 
 loids ; w r hereas when we come nearer to the sun, they will con¬ 
 sist chiefly of metal. Now that being so, we have two oppor¬ 
 tunities of testing the idea which has led to this conclusion. 
 If the exterior planets are metalloidal and the interior planets 
 are metallic — (I do not mean that one group is entirely metal¬ 
 loidal and another entirely metallic)-—it wall be impossible 
 for the density of the exterior planets to be as great as the 
 density of the interior planets. We have already found out 
 that that density, as a matter of fact, is about one to five. 
 
56 
 
 MANCHESTER SCIENCE LECTURES. 
 
 This density, of course, must be lower, because we shall have 
 the best possible conditions for high density where we get 
 the metal in its purest state. Where the density then is least, 
 in the case of the exterior planets, we shall find probably that 
 the velocity of rotation will be different from the velocities 
 of rotation of the other planets. That also, I have already 
 pointed out to you, has been entirely answered by observation. 
 Let us then, if we can, take this one step further. Let us take 
 for granted that the difference between the interior planets 
 and the exterior planets is precisely of this kind. Can we 
 trace the same principle—can we send this chemical touch 
 further into the hypothesis, and deal with, say, no longer the 
 sun and the planet, but a planet and its satellite 1 I think we 
 can; and I think too that in the case of Jupiter, the density 
 of which as compared with the density of its satellites I have 
 already mentioned to you, we have a most singular corroboration 
 of the continuation of the sifting process. 
 
 If the facts I stated in the last lecture were accurate— 
 and I believe they were as accurate as they may be in the 
 present state of science—the density of the satellites of 
 Jupiter in the main is only about one-fifth that of water. 
 Now it so happens that one of the satellites of Jupiter, 
 when it crosses the disc, very often puts on a very dark 
 appearance. We have, therefore, here very good reason to 
 infer that this particular satellite of Jupiter may probably 
 be merely a mass of gas ; and it is very possible that in the 
 case of Neptune and Uranus we may be dealing with 
 bodies which, for the same reason, will remain almost for 
 ever in a semi-gaseous state. And let me remind you that 
 this would at once explain the spectroscopic researches of 
 Yog el and others on the atmospheres of these planets. If 
 you have an atmosphere round Neptune and Uranus consisting 
 of combined metalloids, we may have our choice between 
 several possible spectra. I believe no one has yet made the 
 direct experiment, but the description of the spectrum of 
 Neptune given by Yogel is not very dissimilar from the spectra 
 of the different oxides of nitrogen. That at once then would 
 explain the various spectroscopic differences as well as the 
 compression difference. 
 
 I must remind you, in order to make perfectly clear what I 
 have already ventured to bring before you with regard to the 
 form of these rings, that of course it is assumed that after 
 the ring has been formed it eventually breaks up. Now the 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 57 
 
 facts, I think, seem to show that when this breaking-up takes 
 place, a considerable amount of heat is produced; so that you 
 see, if we consider the density of the exterior planets and their 
 satellites, Jupiter, &c., and of our own Earth and its satellite, 
 we have indications not only that this sifting, this sorting 
 process, did really go on in the formation of the satellites as 
 well as in the formation of the primaries, but , because this process 
 could go on at all , we have , I tlunk , also additional evidence that 
 there must have been a great amount of heat produced . This 
 heat is necessary to the hypothesis, both for the formation of 
 the primary and the secondary bodies; because, unless you 
 have heat enough to get perfect dissociation, you will not have 
 that sorting out which always seems to follow the same law. 
 
 So much then for the passage from star to planet. Does this 
 then throw any light upon the question—what I acknowledge 
 to be one of the great points it is my duty to discuss—of 
 the origin of the chemical constitution of the Earth h 
 
 Twice I have insisted upon this—that the hypothesis is 
 almost worthless unless we assume very high temperatures. 
 Now then, let us consider the Earth, which we know to be one 
 of the interior planets, at the moment the ring—in which 
 form it once existed according to the hypothesis—had been 
 broken. By the breaking of this ring we have the future 
 Earth, a mass of vapour in a state of incandescence. 
 
 I have already told you enough about the near similarity of 
 the chemical constitution of the earth and sun to justify me in 
 again asking you to let me see in the existing sun just such 
 a mass of matter as our Earth then was. Now then, we simply 
 go over the same line of facts again. This list of metals, with 
 the exterior metalloids, must now do duty, not as it did before, 
 for a nebula before it threw off its rings, but for our own 
 Earth, after the time when a ring had formed to become the 
 Moon ultimately. What would happen 1 
 
 We know for certain that the Earth is now a cool body * but 
 I do not think we know that with any greater certainty than 
 we know that the Earth was once a very hot body ; therefore 
 there has been a process of cooling going on. Now I want 
 you to answer for yourselves this question, What would be 
 the stages of the cooling'? We have, by hypothesis, metal¬ 
 loids outside—metalloids not in combination with metals, in 
 consequence of the high temperature. Such as the solar 
 condition is now, such doubtless was the terrestrial condition 
 then. What is the first process then that takes place on 
 
58 
 
 MANCHESTER SCIENCE LECTURES. 
 
 cooling ?—combination. What then will be the process of 
 combination ] The metals and the metalloids will combine, 
 according to their position in the present atmosphere of the 
 sun, in the then atmosphere of the Earth. If, let us say, 
 oxygen and chlorine cannot combine with hydrogen at a 
 particular stage of temperature, they may go lower down, 
 and attempt to combine with calcium and magnesium. But it 
 is certain that if at any time they could have combined with 
 magnesium and calcium, they would have had no chance of 
 combining with iron, because iron was shielded by this 
 enormous buffer of the upper metals. So that we ought to 
 find that the crust of the Earth in the main consists of 
 combinations of metalloids with those metals which exist 
 highest in the atmosphere of the sun. Now it is not necessary 
 that I should state any detailed facts on this point, because 
 Professor Roscoe has already done that for me. I am sure 
 you will all at once grasp that the composition required by 
 the state of things I have pictured is exactly such a com¬ 
 position as Dr. Roscoe has brought before you; and if that is 
 not sufficient I will read this extract from a work by an 
 eminent geologist, Professor Prestwich. This is what he says 
 about the composition of the Earth's crust:— 
 
 “ The whole number of known elements composing the 
 crust and atmosphere of the Earth amount only to sixty-four, 
 and their relative distribution is vastly disproportionate. It 
 has been estimated that oxygen in combination forms by weight 
 one-half of the Earth’s crust; silicon enters for a quarter; 
 then follow aluminium, calcium, magnesium, potassium, 
 sodium, iron, carbon. These nine together have been esti¬ 
 mated to constitute yVoV Earth’s crust. The other 
 
 Tow con sist of the remaining fifty-five non-metallic and 
 metallic elements.” 1 
 
 Now I think that I shall carry you with me when I say that 
 an hypothesis like the one which I have brought before you 
 —an hypothesis due to Kant and Laplace—which leads to 
 mental results so very near the truth of nature, if it cannot 
 be accepted as actual truth, is so near the truth that it 
 deserves that any amount of trouble shall be brought to bear 
 upon it to test it more and more carefully, to find out its 
 weak points and to promulgate its strong ones, in order to 
 beget thorough work. 
 
 You see then that if the metalloids combine with this 
 1 Prestwich, The Past and Future of Geology. Macmillan, 1875. 
 
WHY THE EARTH’S CHEMISTRY IS AS IT IS. 
 
 59 
 
 upper group of metals, two things happen—two points rela¬ 
 tive to the chemistry of the Earth, which are well Worth 
 further inquiry. It follows that a large mass of the 
 interior of the Earth consists of pure metal - and as Dr. 
 Roscoe has referred to that point I need not touch upon it 
 further—and if the Earth is anything like the sun—and we 
 have no reason to doubt their almost perfect similarity — we 
 may be perfectly certain that a large portion of that metal will 
 be iron. It is quite possible that the magnetic state of the 
 Earth—for the Earth is a great magnet—may be connected 
 with the existence of this enormous mass of perfectly pure 
 iron low down in the Earth. I am not recalling your 
 attention to the existence of this pure iron with any commer¬ 
 cial view, but it is important that we should learn all we can 
 in regard to the Earth’s magnetism, upon which our commerce 
 so largely depends. 
 
 Another point is this, if the history of the Earth and the 
 history of the other planets, so far as the chemical nature 
 of the crusts goes, is a history of the descent of the metalloids 
 upon the metals, we must regard the atmosphere of any planet 
 at any time as the mere residuum which has been left after all 
 possible combination has taken place. Our own atmosphere 
 consists of oxygen and nitrogen—still uncombined, fortu¬ 
 nately—and of aqueous vapour, a combination of oxygen with 
 the highest known metal which we see on the list. 
 
 This consideration at once explains how it is that in the 
 moon—which at one time doubtless was the scene of activities 
 of a geological character probably far greater than anything 
 which took place here; that in the moon—of which I bring 
 before you a very beautiful portrait by Mr. Ruth erf urd, of 
 New York—so far as we know, there is now no atmosphere at 
 all; or, at all events, if any atmosphere exist there, it is one 
 so slight that scarcely a remnant of the pure gases has been 
 left, and no aqueous vapour. 
 
 This brings us to several very interesting questions re 
 garding the Earth’s future ; but I feel that I shall not be 
 justified in entering upon them. I trust, however, with regard 
 to the past, that you will be content to see, in the facts that 
 I have ventured to bring before you, that chemists really 
 would be justified in giving great attention to the study of 
 the chemical nature of non-terrestrial matter, in order that 
 the true chemistry of the Earth may be better understood. 
 
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