since these lectures were delivered in the royal institution of great britain there has been much advance in our knowledge of astronomy. the simultaneous advance in other sciences allied with astronomy has been, perhaps, even more remarkable. i am glad to avail myself of the opportunity afforded by a new issue of “the earth’s beginning” to draw attention to certain recent developments of science which relate in a very striking way to the subject of this volume, namely, the famous nebular theory of the origin of the solar system. it appears to me that these recent developments tend to reduce greatly, even if they do not altogether remove, the chief outstanding difficulty which has hitherto retarded the acceptance of the nebular theory.
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 contraction of the prim?val nebula—and this was the supposition of helmholtz—the orb of day cannot have radiated with its present intensity 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 abode 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 phenomena.
there is another entirely different line of reasoning by which professor john joly has recently taught us the immense antiquity 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 previously been sustained in the great vapours of the atmosphere, were deposited in the ocean beds. when the earth had become sufficiently cool to permit of the vapours now forming the ocean passing from the gaseous to the liquid form, the oceans descended from the heavens above to the earth beneath in the form of fresh water. in the lapse 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 evaporation, 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 uncertainty 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 salinity of river water. we have also made a large assumption in supposing that we have accurately 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 inevitable. 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 astronomical 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 discrepancy. science demands that some reconciliation must be effected, yet how is that to be accomplished? 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 insignificant. 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 boundless, 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 velocities enormously greater than those ixof any celestial body with which astronomy had made us acquainted. the case of high corpuscular velocity which is most generally known is that presented by radium, the particles from which are being continually shot forth in myriads. 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 unity followed by eighteen cyphers, or more concisely 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 virtue 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 combustion 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 protract 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 vanquished. thus, the discovery of radium, and the wonderful phenomena associated therewith, has pointed 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 fixed 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 portraiture, the importance of the spiral structure in nebul? becomes of ever-increasing interest.
but i revert 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 milky 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 justify 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 densities of its different parts, would, as shown by mr. easton, be completely xiaccounted for if the milky way were a mighty 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 galaxy which lies in cygnus.
thus, the significance of the spiral structure in the universe becomes greatly enhanced. the spirals abound 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 myriad 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 constituent 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.
