I have explained in Chapter VI. those views of Helmholtz which have for so long provided the received explanation of the maintenance of solar heat. Calculation shows that if the sun’s heat has been maintained by the contraction6 of the prim7?val nebula4—and this was the supposition of Helmholtz—the orb8 of day cannot have radiated with its present intensity9 for a period much longer than twenty million years.
But from the evidence of geology it must now be viadmitted that the existence of our earth, indeed even that part of its existence during which it has been the abode10 of life, has endured for a period far in excess of that which this calculation would allow. It therefore seems to follow that the theory of Helmholtz does not provide an adequate explanation of such an amazing phenomenon as the continuance of a sufficient supply of sunbeams throughout the vast periods demanded by geological phenomena11.
There is another entirely12 different line of reasoning by which Professor John Joly has recently taught us the immense antiquity13 of our earth. His argument is based upon an estimate of the time that must have elapsed since the waters of the ocean, which had previously15 been sustained in the great vapours of the atmosphere, were deposited in the ocean beds. When the earth had become sufficiently16 cool to permit of the vapours now forming the ocean passing from the gaseous17 to the liquid form, the oceans descended18 from the heavens above to the earth beneath in the form of fresh water. In the lapse14 of subsequent ages the sea has become salt because ordinary river water, which always contains some small quantity of salt in solution, is continually bearing salt down to the sea. No doubt water is constantly being abstracted from the sea by evaporation19, but only fresh water is thus removed, so in this cycle of change the salt in the sea must be gradually accumulating. Thus, day by day, though no doubt extremely slowly, the sea has been growing more and more salt.
Professor Joly has made an estimate of the quantity of salt daily added to the sea by all the rivers of the globe. He has also made an estimate of the viitotal quantity of salt which is at present contained in the sea. He has thus the means of forming an estimate of the number of years necessary for the sea to have become converted from its prim?val freshness to its present saltness. His result is not a little astonishing. The saltness of the sea could not be accounted for unless the rivers had been running into the sea for at least a hundred million years. This period is five times as long as the total period during which the sun could have been shining if the Helmholtzian view were correct.
Of course, there are many elements of uncertainty20 in such a calculation. We have assumed that the total flow of the rivers is practically constant, and that our estimate fairly represents the average salinity21 of river water. We have also made a large assumption in supposing that we have accurately22 estimated the total volume of salt in the oceans. But taken in conjunction with the geological evidence already referred to, taken in conjunction with the immense periods of time that have been required for the evolution of life on the globe by the process of natural selection, the conclusion arrived at is inevitable23. It seems impossible to doubt that the sun must have been shining and that our solar system must have existed in practically the same form as it is at present for periods enormously greater than would have been possible if the heat of the sun had been sustained by the solar contraction only.
The difficulty here indicated has been not unjustly considered the most serious difficulty with which the development of modern physical and astronomical24 science has been confronted. The time during which viiithe sun must have lasted, according to the received explanation of the source of its heat and the time during which the sun has actually lasted, as shown by the facts of geology, present a wide discrepancy25. Science demands that some reconciliation26 must be effected, yet how is that to be accomplished27? There is only one possible solution of the problem. It is obvious that there must have been some vast reserve of heat in the sun in comparison with which the quantity of heat yielded by the contraction may be deemed insignificant28. Until this new source of solar energy had been discovered, our knowledge of the physics of the solar system lay under a reproach, which it was the bounden duty of men of science to endeavour to remove.
During the last few years lines of research carried on in various directions have, in a most unexpected manner, thrown much light on the origin of the sun’s heat, and, indeed, we may now say that the great difficulty which has for so long troubled us no longer exists in a serious form.
Recent discoveries show that matter possesses stores of energy which, if not actually boundless29, are enormously in excess of what had been previously deemed possible. These stores of energy are available for supplying the heat of the sun, and it is easy to show that they are amply sufficient to furnish the necessary sunbeams for even the longest periods during which the claims of geology maintain that the sun must have been shining.
The researches of Professor Sir J. J. Thomson have shown how corpuscles of matter are sometimes moving with velocities30 enormously greater than those ixof any celestial31 body with which astronomy had made us acquainted. The case of high corpuscular velocity32 which is most generally known is that presented by radium, the particles from which are being continually shot forth33 in myriads34. It is quite true that each of these corpuscles is excessively small, and it may be useful to give the following illustration bearing on the subject. Think of a number represented by unity3 followed by eighteen cyphers, or more concisely36 as 1018, and think of a line a kilometre long. If that line were divided into 1018 parts, each of those parts would represent the diameter of a corpuscle of radium. If that line were multiplied by 1018, the result would be a line so long that a ray of light would require a period of no less than 100,000 years to pass from one end to the other.
These corpuscles of radium are, no doubt, excessively small, but the velocity with which they are moving is comparable with the velocity of light. When a material object is moving with a velocity of that magnitude the energy it contains in virtue37 of that velocity is indeed startling. A very small grain of sand would, if moving with the velocity of light, contain, in virtue of that motion, the equivalent of more heat than could be produced by the combustion38 of a ton of the best coal. The late Dr. W. E. Wilson showed that if an excessively minute percentage of radium should be found to exist in the sun, it would completely account for the sustentation of the solar heat, and the Hon. R. Strutt has shown that the minute quantities of radium which he has proved to exist in terrestrial rocks would enormously protract39 the earth’s cooling. These discoveries have, in fact, xcompletely changed the outlook on the problem of the sun’s heat, and, though no doubt much has yet to be done before the whole subject is cleared up, the great difficulty may be regarded as vanquished40. Thus, the discovery of radium, and the wonderful phenomena associated therewith, has pointed41 out a possible escape from one of the gravest difficulties in science.
The most notable fact which emerges from the modern study of the structure of the heavens is the ever-increasing significance and importance of the spiral nebul?. The following pages will have failed in their object if they have not succeeded in emphasising the fact that the spiral nebula is, next to a fixed42 star itself, the most characteristic type of object in the material universe. With every increase in the power of the telescope, and with every development of the application of photography to celestial portraiture43, the importance of the spiral structure in nebul? becomes of ever-increasing interest.
But I revert44 to this subject here for the purpose of taking notice of a suggestive paper by Mr. C. Easton in the “Astrophysical Journal,” Vol. XII., No. 2, September, 1900, entitled “A New Theory of the Milky45 Way.” This paper advances the striking view that the Milky Way is itself a spiral nebula, and certainly the considerations adduced by Mr. Easton seem to justify46 his remarkable conclusion.
It is first to be noticed that the Milky Way extends as an irregular band completely round the heavens, and that it follows very nearly the course of a great circle. The curious convolutions of the Milky Way, the varying star densities47 of its different parts, would, as shown by Mr. Easton, be completely xiaccounted for if the Milky Way were a mighty48 spiral. We view the ordinary celestial spirals from the outside at an immense distance in space. We view the Milky Way from a position within the circuit of the nebula. It has, however, been shown by Mr. Easton that the centre of the Spiral Nebula is not exactly at the sun. The centre of the Milky Way is near that superb region of the galaxy49 which lies in Cygnus.
Thus, the significance of the spiral structure in the universe becomes greatly enhanced. The spirals abound50 in every part of the heavens; they are placed in every conceivable position and in every possible plane; they have every range in size from comparatively small objects, whose destiny is to evolve into a system like our solar system, up to stupendous objects which include a myriad35 of such systems. There is now the further interest that as the sun and the solar system are included within the Milky Way, and as the Milky Way is a spiral, this earth of ours is itself at this moment a constituent51 part of a great spiral.
Finally, I would say that, so far as I have been able to understand the subject, it appears to me that every advance in our knowledge of the heavens tends more and more to support the grand outlines of the Nebular Theory as imagined by Kant and Laplace.
R. S. B.
May 1, 1909.
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1
allied
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| adj.协约国的；同盟国的 | |
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remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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unity
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| n.团结，联合，统一；和睦，协调 | |
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nebula
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| n.星云，喷雾剂 | |
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retarded
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| a.智力迟钝的，智力发育迟缓的 | |
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contraction
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| n.缩略词，缩写式，害病 | |
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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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orb
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| n.太阳；星球；v.弄圆；成球形 | |
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intensity
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| n.强烈，剧烈；强度；烈度 | |
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abode
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| n.住处，住所 | |
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phenomena
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| n.现象 | |
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12
entirely
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| ad.全部地，完整地；完全地，彻底地 | |
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antiquity
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| n.古老；高龄；古物，古迹 | |
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lapse
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| n.过失，流逝，失效，抛弃信仰，间隔；vi.堕落，停止，失效，流逝；vt.使失效 | |
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15
previously
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| adv.以前，先前（地） | |
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sufficiently
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| adv.足够地，充分地 | |
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gaseous
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| adj.气体的，气态的 | |
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descended
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| a.为...后裔的，出身于...的 | |
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evaporation
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| n.蒸发，消失 | |
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uncertainty
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| n.易变，靠不住，不确知，不确定的事物 | |
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salinity
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| n.盐分；咸度；盐浓度；咸性 | |
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accurately
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| adv.准确地，精确地 | |
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inevitable
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| adj.不可避免的，必然发生的 | |
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astronomical
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| adj.天文学的，（数字）极大的 | |
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discrepancy
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| n.不同；不符；差异；矛盾 | |
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reconciliation
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| n.和解，和谐，一致 | |
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accomplished
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| adj.有才艺的；有造诣的；达到了的 | |
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insignificant
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| adj.无关紧要的，可忽略的，无意义的 | |
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boundless
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| adj.无限的；无边无际的；巨大的 | |
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velocities
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| n.速度( velocity的名词复数 )；高速，快速 | |
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celestial
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| adj.天体的；天上的 | |
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velocity
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| n.速度，速率 | |
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forth
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| adv.向前；向外，往外 | |
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myriads
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| n.无数，极大数量( myriad的名词复数 ) | |
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myriad
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| adj.无数的；n.无数，极大数量 | |
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concisely
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| adv.简明地 | |
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virtue
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| n.德行，美德；贞操；优点；功效，效力 | |
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combustion
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| n.燃烧；氧化；骚动 | |
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protract
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| v.延长，拖长 | |
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vanquished
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| v.征服( vanquish的过去式和过去分词 )；战胜；克服；抑制 | |
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pointed
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| adj.尖的，直截了当的 | |
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fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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43
portraiture
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| n.肖像画法 | |
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44
revert
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| v.恢复，复归，回到 | |
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45
milky
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| adj.牛奶的，多奶的；乳白色的 | |
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justify
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| vt.证明…正当(或有理)，为…辩护 | |
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47
densities
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| 密集( density的名词复数 )； 稠密； 密度（固体、液体或气体单位体积的质量）； 密度（磁盘存贮数据的可用空间） | |
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48
mighty
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| adj.强有力的；巨大的 | |
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galaxy
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| n.星系；银河系；一群(杰出或著名的人物) | |
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50
abound
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| vi.大量存在；(in，with)充满，富于 | |
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51
constituent
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| n.选民；成分，组分；adj.组成的，构成的 | |
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