WE pointed5 out in the last chapter how, in consequence of its perennial6 loss of heat, the orb7 of day must be undergoing a gradual diminution8 in size. In the present chapter we are to set down the remarkable9 conclusions with respect to the early history of the sun to which we have been conducted by pursuing to its legitimate10 consequences the shrinkage which the sun had undergone in times past.
The outer circle in Fig11. 19 represents the track in which our earth now revolves12 around the sun, and we are to understand that the radius13 of this circle is about ninety-three million miles. We must imagine that the innermost of the four circles represents the position of the sun. Along its track the earth revolves year after year; so it has revolved14 for centuries, so it has revolved since the days of the first monarch15 that ever held sway in Britain, so it has revolved during all the time over which history extends, so it has doubtless revolved for 113illimitable periods anterior16 to history. For an interval17 of time that no one presumes to define with any accuracy the earth has revolved in the same track round that sun in heaven which, during all those ages, has dispensed19 its benefits of light and heat for the sustenance20 of life on our globe.
Fig. 19.—To Illustrate21 the History of the Sun.
Present orbit of Earth.
Sun in times very much earlier still.
Sun in very early times.
Present Sun.
The sun appears constant during those few years in which man is allowed to strut22 his little hour. The size of the sun and the lustre23 of the sun has not appreciably24 altered. But the sun does not always remain the same. It has not always shone with the brightness and vigour25 with which it shines now; it will not continue for ever to dispense18 its benefits with the same liberality that it does at present. The sun is always in a state of change. It 114would not indeed be correct to refer to these changes as growths, in the same sense in which we speak of the growth in a tree. Decade after decade the tree waxes greater; but the sun, as we have already explained, does not increase with the time, for the change indeed lies the other way. It may well be that in this present era the sun is near its prime, in so far as its capacity to radiate warmth and brightness is concerned. It is, however, certain that the sun is not now so large as it was in ancient days. The diminution of the orb is still in progress. In these present days of its glorious splendour the orb of day is much larger than it will be in that gloomy old age which destiny assigns to it.
We have already shown how to give numerical precision to our facts. We have stated that the sun’s diameter is diminishing at the rate of one mile every eleven years. We have dwelt upon the remarkable significance of that shrinkage in accounting26 for the sustentation of the sun’s heat. We have now to call on this perennial diminution of the sun’s diameter to provide some information as to the early history of our luminary27.
The innermost circle in our sketch28 is to suggest the sun as it is at present. Millions of years ago the orb of day was as large as I have indicated it by the circle with the words “sun in very early times.” It will, of course, be understood that we do not make any claim to precise representation of the magnitude of the orb. At a period much earlier still, the sun must have been larger still, and we venture so to depict29 it. We know the rate at which the sun is now contracting, and doubtless this rate has continued sensibly unaltered during thousands of years, and indeed we might say scores of thousands of years. But it would not be at all safe to 115assume that the annual rate of change in the sun’s radius has remained the same throughout excessively remote periods in its evolutionary30 history. What we do affirm is, that in the course of its evolution the sun must have been contracting continually, and we have been able to learn the particular rate of contraction31 characteristic of the present time. But though we are ignorant of the rate of contraction at very early epochs, yet the sun ever looms32 larger and larger in days earlier and still earlier. But in those early days the sun was not heavier, was not, indeed, quite so heavy as it is at present. For we remember that the sun is perennially33 adding thousands of tons to its bulk by the influx34 of meteors. Perhaps we ought to add that the gain of mass from the meteors may be to some extent compensated35 by the loss of substance which the sun not infrequently experiences if, as is sometimes supposed, it expels in some violent convulsion a mass of material which takes the form of a comet (Fig. 21).
Let us now consider what the density of the sun must have been in those prim?val days, say, for example, when the luminary had ten times the volume that it has at present. Even now, as already stated, it does not weigh half as much again as a globe of water of the same size, so that when it was ten times as big its density must have been only a small fraction of that of water. But we may take a stage still earlier. Let us think of a time—it was, perhaps, many scores of millions of years ago—when the sun was a thousand times as big as it is at present. The same quantity of matter which now constitutes the sun was then expanded over a volume a thousand times greater. A remarkable conclusion follows from this consideration. The air that we breathe has a density which is about the seven-hundredth 116part of that of water. Hence we see that at the time when the materials of the sun were expanded into a volume a thousand times as great as it is at present the density of the luminary must have been about equal to that of ordinary air. We refer, of course, in such statements to the average density of the sun. It will be remembered that the density of the sun cannot be uniform. The mutual36 attractions and pressures of the particles in the interior must make the density greater the nearer we approach to the centre.
We must push our argument further still. We have ascertained37 that the prim?val sun could not have been a dense38 solid body like a ball of metal. It must have been more nearly represented by a ball of gas. There was a time when that collection of matter which now constitutes the sun was so big that a balloon of equal size, filled at ordinary pressure with the lightest of known gases, would contain within it a heavier weight than the sun. At this early period the sun must have been as light as an equal volume of hydrogen. The reasoning which has conducted us to this point remains39 still unimpaired. From that early period we may therefore look back to periods earlier still. We see that the sun must have been ever larger and larger, for the same quantity of material must have been ever more and more diffused40. There was a time when the mean density of the sun must have been far less than that of the gas in any balloon.
We must not pause to consider intermediate stages. We shall look back at once to an excessively early period when the sun—or perhaps we ought rather to say the matter which in a more 117condensed form now constitutes the sun—was expanded throughout the volume of a globe whose radius was as great as the present distance from the sun to the earth. Have we not here truly an astonishing result, deduced as a necessary consequence from the fundamental laws of heat?
(Photographed during Eclipse by Professor W. H. Pickering.)
I need hardly say that the sun at that early date did not at all resemble the glorious orb to which we owe our very existence. The prim?val sun must have been a totally different object, as we can easily imagine if we try to think that the sun’s 118materials then filled a volume twelve million times as great as they occupy at present. Instead of comparing such an object with the gases in our ordinary atmosphere, it should rather be likened to the residue43 left in an exhausted44 receiver after the resources of chemistry have been taxed to make as near an approach as possible to a perfect vacuum.
We can give a familiar illustration of gas in a state of extreme tenuity. Look at the beautiful incandescent45 light with which in these days our buildings are illuminated46. How brilliantly those little globes shine! The globe has to be most carefully sealed against the outside air. If there were the smallest opportunity for access, the air from outside would rush in and the lamp would be destroyed. In the preparation of such a lamp elaborate precautions have to be taken to secure that the exhaustion48 of the air from the little globe shall be as nearly perfect as possible. Of course it is impossible to remove all the air. No known processes can produce a perfect vacuum. Some traces of gas would remain after the air-pump had been applied49 even for hours.
We must now imagine a globe, not merely two inches in diameter like one of these little lamps, but a globe 186,000,000 miles in diameter, a globe so large that the earth’s orbit would just form a girdle round it. Even if this globe had been exhausted, so that its density was only the twelve-thousandth part of the ordinary atmospheric50 density, it would still contain more material than is found in the sun in heaven. Thus our reasoning has conducted us to the notion of an epoch1 when the sun—or rather I 119should say the matter composing the sun—formed something totally different from the orb which we know so well. The matter in that very diffuse41 state would not dispense light and heat as a sun in the sense in which we understand the word. However vast might be the store of energy which it contained—a store indeed thousands of times greater than our present sun possesses—yet it would hardly be possessed51 of the power of effective radiation. It would assuredly not be able to warm and light a world associated with it, in the same way as the sun now provides so gloriously for our wants and comfort.
Fig. 21.—The Great Comet of 1882.
(Photographed on November 7th, 1882, by Sir David Gill, K.C.B.)
But it is certain that in those early days there was no earth to be warmed and lighted. Our globe, even if 120it can be said to have existed at all, was truly “without form and void.” At the time when the sun was swollen52 into a great globe of gas or rarefied matter, the elementary substances which were to form the future earth were in a condition utterly53 different from that of our present globe. The history of this earth itself involves another chapter of the argument. Let it suffice to notice, for the present, that our reasoning has led us to a time when the sun consisted only of a rarefied gaseous54 material, and let us give to the matter in this condition the name which astronomers55 apply to any object of a similar character wherever they may meet with it in the universe. Suppose that we could observe through our telescopes at the present moment an object in remote space which was like what the sun must have been at that early stage of its existence which we have been considering, I do not think that the object would be unfamiliar56 to astronomers. There is, indeed, no doubt that there are many objects visible at this moment, and nightly studied in our observatories57, which are formed of matter just in the same state as the sun was in those early times. Examined with a good telescope, the object would seem like a small stain of light on the black background of the sky. The observer would at once call it a nebula. In these modern days he would probably apply the spectroscope to it, and this instrument would assure him that the object he was looking at was a mass of incandescent gas. Such an object would in all probability not greatly differ from many nebul? now known to us.
This being so, why should we withhold58 from the sun of primitive59 days the designation to which it seems to 121be so fully47 entitled? Why should we not speak of it as a nebula? The application of the laws of heat has shown that the great orb of day was once one of those numerous objects which astronomers know as nebul?, and perhaps it may not be too fanciful to suppose that a trace of the prim?val nebula still survives in what we call the Solar Corona (Fig. 20).
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1
epoch
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| n.(新)时代；历元 | |
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prim
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| adj.拘泥形式的，一本正经的；n.循规蹈矩，整洁；adv.循规蹈矩地，整洁地 | |
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3
density
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| n.密集，密度，浓度 | |
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4
nebula
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| n.星云，喷雾剂 | |
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5
pointed
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| adj.尖的，直截了当的 | |
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6
perennial
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| adj.终年的；长久的 | |
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7
orb
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| n.太阳；星球；v.弄圆；成球形 | |
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8
diminution
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| n.减少；变小 | |
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9
remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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10
legitimate
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| adj.合法的，合理的，合乎逻辑的；v.使合法 | |
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11
fig
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| n.无花果(树) | |
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12
revolves
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| v.（使）旋转( revolve的第三人称单数 )；细想 | |
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radius
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| n.半径，半径范围；有效航程，范围，界限 | |
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14
revolved
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| v.（使）旋转( revolve的过去式和过去分词 )；细想 | |
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15
monarch
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| n.帝王，君主，最高统治者 | |
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16
anterior
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| adj.较早的；在前的 | |
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17
interval
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| n.间隔，间距；幕间休息，中场休息 | |
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18
dispense
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| vt.分配，分发；配(药)，发(药)；实施 | |
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19
dispensed
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| v.分配( dispense的过去式和过去分词 )；施与；配（药） | |
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20
sustenance
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| n.食物，粮食；生活资料；生计 | |
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illustrate
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| v.举例说明，阐明；图解，加插图 | |
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22
strut
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| v.肿胀，鼓起；大摇大摆地走；炫耀；支撑；撑开；n.高视阔步；支柱，撑杆 | |
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23
lustre
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| n.光亮，光泽；荣誉 | |
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24
appreciably
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| adv.相当大地 | |
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25
vigour
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| （=vigor）n.智力，体力，精力 | |
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26
accounting
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| n.会计，会计学，借贷对照表 | |
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27
luminary
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| n.名人，天体 | |
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28
sketch
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| n.草图；梗概；素描；v.素描；概述 | |
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29
depict
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| vt.描画，描绘；描写，描述 | |
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30
evolutionary
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| adj.进化的；演化的，演变的；[生]进化论的 | |
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contraction
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| n.缩略词，缩写式，害病 | |
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32
looms
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| n.织布机( loom的名词复数 )v.隐约出现，阴森地逼近( loom的第三人称单数 )；隐约出现，阴森地逼近 | |
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33
perennially
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| adv.经常出现地；长期地；持久地；永久地 | |
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34
influx
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| n.流入，注入 | |
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35
compensated
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| 补偿，报酬( compensate的过去式和过去分词 )； 给（某人）赔偿（或赔款） | |
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mutual
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| adj.相互的，彼此的；共同的，共有的 | |
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ascertained
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| v.弄清，确定，查明( ascertain的过去式和过去分词 ) | |
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38
dense
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| a.密集的，稠密的，浓密的；密度大的 | |
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39
remains
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| n.剩余物，残留物；遗体，遗迹 | |
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40
diffused
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| 散布的，普及的，扩散的 | |
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41
diffuse
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| v.扩散；传播；adj.冗长的；四散的，弥漫的 | |
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42
corona
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| n.日冕 | |
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43
residue
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| n.残余，剩余，残渣 | |
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44
exhausted
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| adj.极其疲惫的，精疲力尽的 | |
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45
incandescent
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| adj.遇热发光的， 白炽的，感情强烈的 | |
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46
illuminated
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| adj.被照明的；受启迪的 | |
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47
fully
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| adv.完全地，全部地，彻底地；充分地 | |
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48
exhaustion
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| n.耗尽枯竭，疲惫，筋疲力尽，竭尽，详尽无遗的论述 | |
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49
applied
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| adj.应用的；v.应用，适用 | |
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50
atmospheric
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| adj.大气的，空气的；大气层的；大气所引起的 | |
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51
possessed
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| adj.疯狂的；拥有的，占有的 | |
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52
swollen
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| adj.肿大的，水涨的；v.使变大，肿胀 | |
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53
utterly
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| adv.完全地，绝对地 | |
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54
gaseous
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| adj.气体的，气态的 | |
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55
astronomers
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| n.天文学者，天文学家( astronomer的名词复数 ) | |
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56
unfamiliar
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| adj.陌生的，不熟悉的 | |
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57
observatories
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| n.天文台，气象台( observatory的名词复数 ) | |
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58
withhold
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| v.拒绝，不给；使停止，阻挡 | |
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59
primitive
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| adj.原始的；简单的；n.原(始)人，原始事物 | |
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