nebula which extended so far out as to fill all the space at present
occupied by the planets. This gigantic nebulous mass, of
which the sun was only the central and somewhat more condensed
portion, is supposed to have a movement of rotation on
its axis. There is no difficulty in conceiving how a nebula,
quite independently of any internal motion of its parts, shall
also have had as a whole a movement of rotation. In fact a
little consideration of the theory of probabilities will show it
to be infinitely probable that such an object should really have
some movement of rotation, no matter by what causes the
nebula may have originated. As this vast mass cooled it
must by the laws of heat have contracted towards the centre,
and as it contracted it must, according to a law of dynamics,
rotate more rapidly. The time would then come when the
centrifugal force on the outer parts of the mass would more
than counterbalance the attraction of the centre, and thus
we would have the outer parts left as a ring. The inner portion
will still continue to contract, the same process will be repeated,
and thus a second ring will be formed. We have thus grounds
for believing that the original nebula will separate into a series
of rings all revolving in the same direction with a central nebulous
mass in the interior. The materials of each ring would continue
to cool and to contract until they passed from the gaseous to
the liquid condition. If the consolidation took place with
comparative uniformity we might then anticipate the formation
of a vast multitude of small planets such as those we actually
do find in the region between the orbit of Mars and that of
Jupiter. More usually, however, the ring might be expected
not to be uniform, and, therefore, to condense in some parts
more rapidly than in others. The effect of such contraction
would be to draw the materials of the ring into a single mass,
and thus we would have a planet formed, while the satellites
of that planet would be developed from the still nascent planet
in the same way as the planet itself originated from the sun.
In this way we account most simply for the uniformity in the
direction in which the planets revolve, and for the mutual
proximity of the planes in which their orbits are contained.
Such was the nebular theory as it was originally sketched. At the present day when the nebulae that are spiral in form have been shown to be so numerous, next to the fixed stars themselves, our view of the nebular theory has been somewhat modified. It now seems probable that the spiral nebula is the fittest illustration of the transformation of a diffused nebula into a system of sun and planets.
The rotation of the planets on their axes is also explained as a consequence of the nebular theory, for at the time of the first formation of the planet it must have participated in the rotation of the whole nebula, and by the subsequent contraction of the planet the speed with which the rotation was performed must have been accelerated.
There is quite a different method of considering the nebular origin of our system, which leads in a very striking manner to conclusions practically identical with those we have just sketched. We may commence by dealing with the sun as we find it at the present moment, and thence inferring what must have been the progress of events in the earlier epochs of the history of our system.
The daily outpour of heat from the sun at the present time suggests a profound argument in support of the nebular theory. The amount of the sun’s heat has been estimated, but we receive on the earth less than one two-thousand-millionth part of the whole radiation. It would seem that the greater part of the rest flows away to be lost in space. Now what supplies this heat? We might at first suppose that the sun was really an intensely heated body radiating out its heat as does white-hot iron, but this explanation cannot be admitted, for there is no historical evidence that the sun is growing colder. We have not the slightest reason to think that the radiation from the sun is measurably weaker now than it was a couple of thousand years ago, yet it can be shown that, if the sun were merely radiating heat as simply a hot body, then it would cool some degrees every year, and must have cooled many thousands of degrees within the time covered by historical records. We, therefore, conclude that the sun has some other source of heat than that due simply to incandescence. It might, for example, be suggested that the heat of the sun was supplied by chemical combination analogous to combustion. It would take 20 tons of coal a day burned on each square foot of the sun’s surface to supply the daily radiation. Even if the sun were made of one mass of fuel as efficient as coal, that mass must be entirely expended in a few thousand years if the present rate of radiation was to be sustained. We cannot, therefore, admit that the source of the heat in the sun is to be found in any chemical combination taking place in its mass. Where then can we find an adequate supply of heat? Only one external source can be named: the falling of meteors into the sun must yield some heat just as a shooting star yields some heat to our atmosphere, but the question is whether the quantity of heat obtainable from the shooting stars is at all adequate for the purpose. It can be shown that unless a quantity of meteors in collective mass equal to our moon were to plunge into the sun every year the supply of heat could not be sustained from this source. Now there is no reason to believe that meteors in anything like this quantity can be supplied to the sun, and, therefore, we must reject this source as also inadequate.
The truth about the sun’s heat appears to be that the sun is really an incandescent body losing heat, but that the operation of cooling is immensely retarded owing to a curious circumstance due jointly to the enormous mass of the sun and to a remarkable law of heat. It is well known that if energy disappears in one form it reappears in another, and this principle applied to the sun will explain the famous difficulty.
As the sun loses heat it contracts, and every pair of particles in the sun are nearer to each other after the contraction than they were before. The energy due to their separation is thus less in the contracted state than in the original state, and as that energy cannot be lost it must reappear in heat. The sun is thus slowly contracting; but as it contracts it gains heat by the operation of the law just referred to, and thus the further cooling and further contraction of the sun is protracted until the additional heat obtained is radiated away. In this way we can reconcile the fact that the sun is certainly losing heat with the fact that the change in temperature has not been large enough to be perceived within historic times.
It has been estimated that the sun is at present contracting so that its diameter diminishes 10 m. every century; there is, however, now reason to think that the rate of contraction is by no means so rapid as this would indicate. This is an inappreciable distance when compared with the diameter of the sun, which is nearly a million of miles, but the significance for our present purpose depends upon the fact that this contraction is always taking place. Assuming the accuracy of the estimate just made, we see that a thousand years ago the sun must have had a diameter 100 m. greater than at present, ten thousand years ago that diameter must have been 1000 m. more than it is now, and so on. We cannot perhaps assert that the same rate is to be continued for very many centuries, but it is plain that the further we look back into the past time the greater must the sun have been.
Dealing then simply with the laws of nature as we know them, we can see no limit to the increasing size of the sun as we look back. We must conceive a time when the sun was swollen to such an extent that it filled up the entire space girdled by the orbit of Mercury. Earlier still the sun must have reached to the earth. Earlier still the sun must have reached to where Neptune now revolves on the confines of our system, but the mass of the sun could not undergo an expansion so prodigious without being made vastly more rarefied than at present, and hence we are led by this mode of reasoning to the conception of the primaeval nebula from which our system has originated.
Considering that our sun is but a star, or but one of the millions of stars, it is of interest to see Whether any other systems present indication of a nebulous origin analogous to that which Laplace
