evidence against all other theories but one, and that one theory is confirmed by line after line of positive reasoning. To doubt what general view we should form of the corona and zodiacal light under these circumstances seems to me to savour-not of that wise caution which prevents the true philosopher from overlooking difficulties, but rather-of an inaptitude to estimate the value of evidence. As to details we may be doubtful. Other matter than meteoric or cometic matter may well be in question; other modes of producing light, save heat, electricity, or direct illumination, may be in operation in this case; and lastly there may be other forces at work than the attractive influence of solar gravity, or the form of repulsive force evidenced by the phenomena of comets. regards, also, the true shape and position of the coronal and zodiacal appendage-and yet more as regards its variations in shape-we may still have much to learn. But of the general fact that the corona and zodiacal light form a solar appendage of amazing extent and importance, that they are not merely terrestrial phenomena, but worthy of all the attention astronomers and physicists can direct to them, it seems to me that no reasonable doubts can any longer be entertained.

As

CHAPTER VII.

PHYSICAL CONDITION OF THE SUN.

IN the course of the last four chapters a number of facts bearing on the physical condition of the Sun have been dealt with at greater or less length. Considered separately these facts are full of interest and seem to afford somewhat satisfactory information on the points they severally relate to. We feel little difficulty, for instance, in giving a general interpretation to the dark lines of the solar spectrum, regarding them as undoubtedly due to the existence of the vapours of certain metallic and other elements in the solar atmosphere. We regard with a certain confidence, again, the concep tion that the spots are depressions of greater or less depth, and further that the light received from the umbra of a spot shines through absorbing vapours, some of which exist at a greater pressure and at a lower temperature than over the rest of the photosphere. We are able, also, to form certain sufficiently definite opinions respecting the prominences, more particularly as regards the pressure at which their substance exists and the motions to which they are subjected. While, lastly, the

corona has been studied with results which cannot but be regarded as trustworthy.

But when we attempt to combine these several results, and further to determine what the general condition of that orb may be which presents these several features, we recognise at once that a problem of enormous difficulty lies before us. The more we have learned respecting the Sun, nay the more we have learned respecting those physical laws by which we are to interpret solar phenomena, the more insuperable have our difficulties become. It was easy to theorise when as yet but little was known. It was easy to suppose that the few physical laws we imagined we understood sufficed to account for all the phenomena presented by the solar orb. But as one fact after another has been discovered, the true complexity of the problem has been revealed to us; and as the physical laws which it is in our power to discuss and experiment on have been more carefully studied, we have begun to recognise how very limited our experience has hitherto been. It is not too much to say that theories respecting the Sun's physical condition which would have been regarded twenty years since as deserving of careful study, have now no worthier standing in science than the idea of Anaximander that the Sun is a great vessel filled with fire, at the top of which is an opening through which the fire escapes.'

6

It is easier to consider those facts which have revealed to us the enormous difficulty of the problem we are upon, than to present any considerations tending to render our conceptions clearer.

In the first place, we recognise the fact that in the Sun the elements exist in conditions altogether different from those we are familiar with. This is true of all orders of elements, from those whose normal condition (as recognised by us) is gaseous, to those which at ordinary or even very high temperatures remain solid or liquid. For instance, metals which we can volatilise indeed in small quantities, and by the aid of special contrivances, but which yet we can only experiment upon (properly speaking) when they are in the solid or liquid state, are present in the Sun as glowing vapours. But also gases which no amount of pressure or refrigeration we can command will cause even to show signs of approaching the liquid condition, probably exist in certain portions of the solar globe as liquids or even as solids. On the one hand, then, we have an inconceivably high temperature volatilising our most fixed elements-nay, for aught we know, perhaps dissociating substances which we regard as elements into their true primary constituents; on the other we have an inconceivably high pressure at a relatively inconsiderable distance beneath the photosphere, reducing our so-called perfect gases into the liquid or solid form.*

*I do not know that we have any sufficient evidence that hydrogen, oxygen, nitrogen, and other gases which we call perfect, are really exceptions to the general rule that all substances are capable of assuming -under suitable conditions--the solid, liquid, and gaseous form. It is true that no pressure or refrigeration we can apply causes these gases to exhibit the least trace of those qualities which the imperfect gases exhibit at ordinary temperatures and pressures. In other words, the perfect gases, as their title implies, obey perfectly (1) the law associating density and pressure; and (2) the law according to which the relation of specific

In the next place, we have learned of late to recognise on how very doubtful a basis many of the received axioms (almost) of physical science have been placed. No laws of science were perhaps more thoroughly accepted than those which were supposed to distinguish the solid, liquid, and gaseous states from each other. All physicists believed that a definite and well-marked change of condition necessarily accompanies the process by which gas passes into the liquid state, and very few were disposed to doubt that a correspondingly distinct line of demarcation separates the liquid from the solid state. Yet the researches of Dr. Andrews have recently shown that under certain conditions carbonic acid gas may be made to pass by absolutely insensible gradations from an undoubtedly gaseous to an undoubtedly liquid state; and it is recognised that what has been proved in the case of a single gas is in all probability true of all gases and vapours. It has been also rendered highly probable that under suitably

But

heats at constant pressure and constant volume remains constant. it is known that imperfect gases at ordinary temperatures and pressures seem very closely to obey these laws, though as they approach more and more nearly to the circumstances under which they become liquid (whether through increased pressure, simple refrigeration, or both combined) they depart in a marked way from these laws. And it is probable that if the imperfect gases could be experimented on at enormously high temperatures and low pressures, they would be found to obey the abovenamed laws as perfectly to all appearance as the perfect gases do. All we can assert respecting the perfect gases is that none of the processes of combined pressure and refrigeration hitherto applied—perhaps none we are capable of applying-bring them even so far towards the state under which they would become liquid as the imperfect gases are brought under ordinary pressures and temperatures.