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great pressure the passage of a molten metal to the solid form, or conversely the melting of such a metal, may take place in a gradual manner, so that at a certain stage of the process it shall be impossible to say whether the metal or certain portions of it be liquid or solid.

Another physical law had been thought to be thoroughly established. It had been supposed that the whiteness (or true incandescence) of flame was due to the presence of minute particles of incandescent matter. But Frankland has shown that with increase of pressure the faintly luminous light of burning hydrogen may be rendered bright, and that by sufficiently increasing the pressure the spectrum of the light becomes continuous. So that what had been supposed the most marked characteristic of incandescent solid and liquid bodies, is thus shown to be a possible characteristic of the light of glowing gas. Thus the whole basis of our reasoning (which had been thought so sound) respecting the actual condition of the solar photosphere, whether as evidenced by direct observation or by analysis with the spectroscope, has been shaken.

Yet again we have learned to recognise the fact that the imagined limits to the rarefaction at which gases may subsist, as such, had been placed far too low. Our experimenters have gone indeed very far towards the production of an actual vacuum, without obtaining any evidence that the almost infinitesimal quantity of gas which can alone remain in the so-called

vacuum-tubes, behaves otherwise than at appreciable pressures.*

It will be seen at once how importantly this bears on the subject of solar physics, since it compels us to reconsider all our ideas respecting the probable limits of atmospheric envelopes; and the most difficult questions of solar physics are precisely those which depend on this matter. Unfortunately the observed fact gives us no new means at present of forming a satisfactory opinion as to atmospheric limits. Even if it should be regarded as demonstrating that there is no real limit to atmospheric extension, we should still be no nearer than before to a determination of the atmospheric pressures at the surface of the Sun or of any planet except our own Earth; and this is the problem which chiefly concerns us here.f

They have carried the rarefaction so far that the electric spark will no longer traverse the tenuous medium; and some have been led to suppose that when this state of things has been brought about there must be an actual vacuum. No such conclusion can be regarded, however, I will not say as demonstrated, but as probable or even conceivable. All the evidence we have tends to show that an absolute vacuum is as imaginary a conception as the philosopher's stone or the perpetual motion. Take the most stable and dense metal, platinum, and conceive that for a moment in the heart of a mass of such metal there was a vacuous space; then in an instant that space would be occupied. For, even setting aside the probability that the most solid metals undergo an indefinitely minute but still real dissipation at their surfaces, corresponding to that dissipation which ice undergoes at the lowest temperatures yet experimented onsetting aside, I say, this probability, there yet remains the certainty that in the intimate molecular structure of the platinum a perfectly free communication exists between the imagined space within and the space without. The communication is not, indeed, such as suffices for the conveyance of certain forms of matter or of motion; but that it is none the less real on that account cannot be questioned.

† It would, indeed, be pleasant to theorise on this matter,—to conclude,

Still further, in considering the Sun's physical condition, we have to discuss the effect of motions wholly surpassing in velocity any that we are familiar with on Earth, and therefore à fortiori any that we can experiment upon. Setting aside the fact that matter from without must continually be falling upon the Sun (in whatever condition) with velocities such as have been dealt with in Chapter II., we have the observed fact that movements fairly comparable with these have been recognised in the very substance of the Sun by the aid of spectroscopic analysis. Now, what means have we for determining the probable effect of motions of 100 miles per second taking place even among substances in conditions such as we are familiar with? We know the effects of certain velocities; we see the bullet melted as it reaches the target, the meteor vaporised as it speeds through the air. But we have no means whatever of determining what effects would be produced by velocities enormously exceeding even the inconceivable velocity of many meteors. When to this we add that the swift motions referred to take place amid depths of vaporised metals, and, for aught we know, over seas and continents of liquefied and solidified gases, we may well shrink from the task of attempting-at least in the

for example, that every planet could have just so much atmosphere as corresponded to the range of its attractive influences; and consequently to deduce the atmospheric pressure upon the Sun. But no confidence could be placed in such theories. For, on the implied supposition, quite other forces than attraction would have to be considered. And, further. it would be an inevitable consequence of such a state of things that as the planets changed their relative positions, the atmospheric pressure at the surface of each would vary.

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present state of our knowledge-to estimate their probable effects.

I confess, therefore, that at this stage of my subject I am very far from sharing that confidence which I find some men possess in dealing with problems of solar physics. I shall not pretend to place all the phenomena in that due order in which they appear in the theories hitherto propounded. I can only look on with a sense of bewildered admiration while the professors of rival theories exhibit the physical habitudes of the Sun as obviously explicable according to contradictory hypotheses. I must admit that it seems to me that only a very energetic forgetfulness of a large proportion of the evidence can account for the adoption of these theories. I must content myself, therefore, with an exceedingly brief statement of certain general relations, which are all that I find satisfactorily exhibited by what has as yet been learned respecting the Sun.

We have in the Sun a vast agglomeration of the elements we are familiar with on Earth; and this vast agglomeration is subject to two giant influences, producing in some sort opposing effects-viz., a temperature far surpassing any we can form any conceptions of, and a pressure (throughout nearly the whole extent of the solar globe) which is perhaps even more disproportionate to the phenomena of our experience. Each known element would (beyond all question) be vaporised by the solar temperature at known pressures; each would (there can be little question) be solidified by the vast solar pressures, did these occur at

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known temperatures. Now, whether under these circumstances the laws of gaseous diffusion prevail where the elements are gaseous in the solar globe; whether where liquid matter exists it is in general bounded in a definite manner from the neighbouring gaseous matter; whether any elements at all are solid, and if so under what conditions their solidity is maintained and the limits of the solid matter defined-all these questions are such as we must answer before we can form a satisfactory view of the solar constitution; and yet they are questions which we have at present no means of answering. Again, we must learn how far combustion, properly so called, can take place within the Sun's mass, and whether those processes which we recognise as combustion are the only processes of combustion which can actually take place there. For aught that is yet known, the intensity of the forces at work upon and within the Sun may wholly prevent the occurrence of any processes of combustion familiar to ourselves; while other processes of true combustion altogether unthought of by us may be in continual action.

Assuming, however, that some general resemblance exists between the processes at work upon the Sun and those we are acquainted with (the wildest assumption possible), we may imagine that the various elements in the solar substance ordinarily exist down to certain definite levels in the gaseous form, at lower levels (definite for each) in the liquid form, at yet lower levels in the solid form. That part of each element which is gaseous must again be divided into two portions that whose light is capable of giving charac

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