The Earth's Beginning
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THE EARTH’S BEGINNING
THE STORY OF THE HEAVENS.
With 24 Coloured Plates and NumerousIllustrations. New Edition. 10s. 6d.
THE EARTH’S BEGINNING.
With 4 Coloured Plates and NumerousIllustrations. New Edition. 7s. 6d.
THE STORY OF THE SUN.
With 11 Full Page Coloured and other Platesand Numerous Illustrations. 7s. 6d.
Being Talks with Young People about theWonders of the Heavens With RembrandtFrontispiece and 94 Illustrations in Text.7s. 6d.
AN ENGLISH SUNSET TINGED BY KRAKATOA.
(From a Drawing made at Chelsea at 4.40 p.m. on Nov. 26th, 1883, by Mr. W. Ascroft.)
Since these lectures were delivered in the RoyalInstitution of Great Britain there has been muchadvance in our knowledge of astronomy. The simultaneousadvance in other sciences allied with astronomyhas been, perhaps, even more remarkable. I amglad to avail myself of the opportunity afforded bya new issue of “The Earth’s Beginning” to drawattention to certain recent developments of sciencewhich relate in a very striking way to the subject ofthis volume, namely, the famous Nebular Theory ofthe origin of the solar system. It appears to methat these recent developments tend to reduce greatly,even if they do not altogether remove, the chief outstandingdifficulty which has hitherto retarded theacceptance of the Nebular Theory.
I have explained in Chapter VI. those views ofHelmholtz which have for so long provided the receivedexplanation of the maintenance of solar heat.Calculation shows that if the sun’s heat has beenmaintained by the contraction of the primæval nebula—andthis was the supposition of Helmholtz—theorb of day cannot have radiated with its presentintensity for a period much longer than twenty millionyears.
But from the evidence of geology it must now beviadmitted that the existence of our earth, indeed eventhat part of its existence during which it has beenthe abode of life, has endured for a period far inexcess of that which this calculation would allow.It therefore seems to follow that the theory of Helmholtzdoes not provide an adequate explanation ofsuch an amazing phenomenon as the continuance ofa sufficient supply of sunbeams throughout the vastperiods demanded by geological phenomena.
There is another entirely different line of reasoningby which Professor John Joly has recently taughtus the immense antiquity of our earth. His argumentis based upon an estimate of the time that must haveelapsed since the waters of the ocean, which had previouslybeen sustained in the great vapours of theatmosphere, were deposited in the ocean beds. Whenthe earth had become sufficiently cool to permit ofthe vapours now forming the ocean passing from thegaseous to the liquid form, the oceans descended fromthe heavens above to the earth beneath in the formof fresh water. In the lapse of subsequent ages thesea has become salt because ordinary river water,which always contains some small quantity of salt insolution, is continually bearing salt down to the sea.No doubt water is constantly being abstracted fromthe sea by evaporation, but only fresh water is thusremoved, so in this cycle of change the salt in thesea must be gradually accumulating. Thus, day byday, though no doubt extremely slowly, the sea hasbeen growing more and more salt.
Professor Joly has made an estimate of the quantityof salt daily added to the sea by all the riversof the globe. He has also made an estimate of theviitotal quantity of salt which is at present contained inthe sea. He has thus the means of forming an estimateof the number of years necessary for the sea to havebecome converted from its primæval freshness to itspresent saltness. His result is not a little astonishing.The saltness of the sea could not be accounted forunless the rivers had been running into the sea for atleast a hundred million years. This period is five timesas long as the total period during which the sun couldhave been shining if the Helmholtzian view werecorrect.
Of course, there are many elements of uncertaintyin such a calculation. We have assumed that thetotal flow of the rivers is practically constant, andthat our estimate fairly represents the average salinityof river water. We have also made a large assumptionin supposing that we have accurately estimatedthe total volume of salt in the oceans. But taken inconjunction with the geological evidence already referredto, taken in conjunction with the immenseperiods of time that have been required for the evolutionof life on the globe by the process of naturalselection, the conclusion arrived at is inevitable. Itseems impossible to doubt that the sun must havebeen shining and that our solar system must haveexisted in practically the same form as it is at presentfor periods enormously greater than would have beenpossible if the heat of the sun had been sustained bythe solar contraction only.
The difficulty here indicated has been not unjustlyconsidered the most serious difficulty with which thedevelopment of modern physical and astronomicalscience has been confronted. The time during whichviiithe sun must have lasted, according to the receivedexplanation of the source of its heat and the timeduring which the sun has actually lasted, as shownby the facts of geology, present a wide discrepancy.Science demands that some reconciliation must beeffected, yet how is that to be accomplished? Thereis only one possible solution of the problem. It isobvious that there must have been some vast reserveof heat in the sun in comparison with which thequantity of heat yielded by the contraction may bedeemed insignificant. Until this new source of solarenergy had been discovered, our knowledge of thephysics of the solar system lay under a reproach,which it was the bounden duty of men of science toendeavour to remove.
During the last few years lines of research carriedon in various directions have, in a most unexpectedmanner, thrown much light on the origin of the sun’sheat, and, indeed, we may now say that the greatdifficulty which has for so long troubled us no longerexists in a serious form.
Recent discoveries show that matter possessesstores of energy which, if not actually boundless, areenormously in excess of what had been previouslydeemed possible. These stores of energy are availablefor supplying the heat of the sun, and it is easy toshow that they are amply sufficient to furnish thenecessary sunbeams for even the longest periods duringwhich the claims of geology maintain that the sunmust have been shining.
The researches of Professor Sir J. J. Thomsonhave shown how corpuscles of matter are sometimesmoving with velocities enormously greater than thoseixof any celestial body with which astronomy had madeus acquainted. The case of high corpuscular velocitywhich is most generally known is that presented byradium, the particles from which are being continuallyshot forth in myriads. It is quite true that each ofthese corpuscles is excessively small, and it may beuseful to give the following illustration bearing onthe subject. Think of a number represented byunity followed by eighteen cyphers, or more conciselyas 1018, and think of a line a kilometre long.If that line were divided into 1018 parts, each of thoseparts would represent the diameter of a corpuscle ofradium. If that line were multiplied by 1018, theresult would be a line so long that a ray of lightwould require a period of no less than 100,000 yearsto pass from one end to the other.
These corpuscles of radium are, no doubt, excessivelysmall, but the velocity with which theyare moving is comparable with the velocity of light.When a material object is moving with a velocityof that magnitude the energy it contains in virtueof that velocity is indeed startling. A very smallgrain of sand would, if moving with the velocity oflight, contain, in virtue of that motion, the equivalentof more heat than could be produced by the combustionof a ton of the best coal. The late Dr. W. E.Wilson showed that if an excessively minute percentageof radium should be found to exist in the sun, itwould completely account for the sustentation of thesolar heat, and the Hon. R. Strutt has shown that theminute quantities of radium which he has proved toexist in terrestrial rocks would enormously protractthe earth’s cooling. These discoveries have, in fact,xcompletely changed the outlook on the problem of thesun’s heat, and, though no doubt much has yet to bedone before the whole subject is cleared up, the greatdifficulty may be regarded as vanquished. Thus, thediscovery of radium, and the wonderful phenomenaassociated therewith, has pointed out a possible escapefrom one of the gravest difficulties in science.
The most notable fact which emerges from themodern study of the structure of the heavens is theever-increasing significance and importance of thespiral nebulæ. The following pages will have failedin their object if they have not succeeded in emphasisingthe fact that the spiral nebula is, next toa fixed star itself, the most characteristic type ofobject in the material universe. With every increasein the power of the telescope, and with every developmentof the application of photography to celestialportraiture, the importance of the spiral structure innebulæ becomes of ever-increasing interest.
But I revert to this subject here for the purposeof taking notice of a suggestive paper by Mr. C.Easton in the “Astrophysical Journal,” Vol. XII.,No. 2, September, 1900, entitled “A New Theory ofthe Milky Way.” This paper advances the strikingview that the Milky Way is itself a spiral nebula,and certainly the considerations adduced by Mr.Easton seem to justify his remarkable conclusion.
It is first to be noticed that the Milky Wayextends as an irregular band completely round theheavens, and that it follows very nearly the courseof a great circle. The curious convolutions of theMilky Way, the varying star densities of its differentparts, would, as shown by Mr. Easton, be completelyxiaccounted for if the Milky Way were a mighty spiral.We view the ordinary celestial spirals from the outsideat an immense distance in space. We view theMilky Way from a position within the circuit of thenebula. It has, however, been shown by Mr. Eastonthat the centre of the Spiral Nebula is not exactlyat the sun. The centre of the Milky Way is near thatsuperb region of the galaxy which lies in Cygnus.
Thus, the significance of the spiral structure inthe universe becomes greatly enhanced. The spiralsabound in every part of the heavens; they are placedin every conceivable position and in every possibleplane; they have every range in size from comparativelysmall objects, whose destiny is to evolve intoa system like our solar system, up to stupendousobjects which include a myriad of such systems.There is now the further interest that as the sunand the solar system are included within the MilkyWay, and as the Milky Way is a spiral,