tion, the latent heat would be developed to an enormous extent, and the meteoric stone would be instantaneously surrounded with an atmospheric furnace.

The same principle would apply to the shooting stars, but being spongy, or even in the form of delicate flakes of combustible materials, they would be not only heated but consumed, just as thin paper would easily take fire when a piece of pasteboard would not be scorched. The products of their combustion would depend on the matter of which they are composed. The circumstance of their never reaching the ground, and sometimes being dissipated in vapour, suggests that probably they were not solid metals like the meteoric stones. At all events, they must have been either dissipated in dust if they were metallic, or melted in air, according to the nature of their materials.

It would appear that, although all space seems sprinkled with these substances, the great store-house of meteoric matter lies within the orbit of our planet, and in the greatest abundance in the immediate vicinity of the sun. Those who have visited tropical countries must have witnessed that faint but beautiful glow of light which appears to rise high in the atmosphere as an immense cone, both in the morning and evening twilight. It is called the zodiacal light, and although involved in considerable uncertainty, it is generally understood to indicate a vast region of meteoric matter revolving round the sun.

There is, therefore, a great probability that in the solar system there exists as much meteoric matter, still floating within its bounds, as would be sufficient to make other bodies, each of them as large as the sun.

CHAPTER VIII.

THE GEOLOGY OF THE SUN.

WE are now prepared to examine how far our knowledge of the composition of the earth and its meteorolites will serve to explain the phenomena of the sun; and it at once appears as if the whole scene, which we have been describing, were presented in reality before our eyes. In the corona that surrounds the sun we behold the atmosphere of oxygen, which is necessary for combustion; and in the zodiacal light we see the store of fuel which is to sustain it, and with which the oxygen is to be combined.

For the sake of the unscientific reader, the explanation will be given in the form of answers to questions which might present themselves as difficulties.

1. Why should the atmosphere round the sun be composed of oxygen?

There are two reasons why we suppose that the atmosphere around the sun is composed of oxygen. First, because we find that our own earth was once surrounded by an atmosphere of almost pure oxygen, which occupied a space in the heavens so large as to form a considerable portion of the solar system. If we suppose the other planets to have been formed in the same manner, we must conclude that the general atmosphere of the solar system is oxygen. The second reason is, that as the sun appears to shine by the combustion of meteoric matter, we could not

suppose that any other atmosphere except oxygen would support its combustion.

2. If the light and heat of the sun be produced by the fall of meteoric matter to its surface, why does not the whole fall

at once?

We find throughout the entire universe an apparent law of rotation or revolution. When two bodies gravitate towards each other, they are always found to revolve round one another. The earth and the planets revolve round the sun, the moons revolve round the planets, and the planets and the sun itself rotate on their own axes. Even the fixed stars (as they are called), when they are double, revolve round one another. We have no reason to suppose, therefore, that the meteoric materials of our system are an exception to the general rule. We have only to suppose that they revolve round the sun as all the other bodies do, and then we have a reason why they do not at once fall to its surface.

3. If the meteoric materials revolve round the sun, why should they ever fall to its surface?

If there be an atmosphere existing throughout the solar system, more especially near the sun, all the bodies that move round it must meet with some resistance in

their course. As regards the planets, this resistance may be so slight as to produce no visible effect, because they are large and heavy; but it is not so with the meteorolites; they present so large a surface, in proportion to their weight, that even a very thin atmosphere would retard their progress, and make them sink towards the sun. It is indeed probable that as their orbits decrease they will become less numerous, because many will unite into one;

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and as it is likely that the heat of the sun will melt them into a globular mass, they will on that account experience less resistance; but, on the other hand, as the atmosphere will always be becoming more dense, the nearer they approach the central body, they will encounter more and more resistance, until at last they plunge into its body.

4. If the meteorolites are for some time revolving round the sun, why do they not take fire from its heat before they fall to its surface?

The heat of the sun is so great that these meteorolites will be melted while they are yet millions of miles away. We shall suppose one to be a million of tons in weight, another a thousand tons, and a third, one ton. We shall also suppose that they are made of the usual materials of meteoric stones, say iron and nickel. When they have approached the sun so as to be melted by its heat, the pure metal, being the heaviest, will sink to the centre of the mass, and the oxides and other materials will rise to the surface, and float upon it, so as to cover the metal and protect it from the atmosphere. But we must recollect that the sun's rays have the power of deoxidising, and therefore when the oxides are exposed to the sun's rays they will be deoxidised, the oxygen will be thrown off, and the pure metal will flow downwards to the centre, leaving fresh oxides on the surface to be decomposed. Instead, therefore, of the meteoric matter being burned by the sun's rays, they would be unburned and restored to their combustible state. We have an illustration of this fact in common fires exposed to the sun; instead of the heat of the sun helping the coal to burn, the deoxidising power of the sun's rays prevents it from burning.

5. How then do they burn when they fall to the sun?

The heat of the sun is sufficient not only to melt, but to boil the metals and other materials of the meteoric stones, that is to say, to convert them into inflammable gas. This does not take place until they have fallen towards the sun's body, and even then, when they are very large, it will take some time before they are all evaporated. The gas which is thus set free forms the interior atmosphere of the sun, which can only burn at the outside where it comes in contact with the oxygen. It is this envelope of flame which we see surrounding the sun's body. On the outside of this flame is the oxygen. In the inside is the inflammable gas, generated by the boiling of the meteoric metals.

We might exhibit a model of this flame by filling a jar with coal gas, and setting fire to it at the mouth. The gas would only burn at the mouth, where it came in contact with the atmospheric air. None of the gas below the flame could burn, until it rose towards the oxygen without.

6. Why should they not boil before they reach the sun? The heat generated by meteorolites is caused, not so much by the combustion of the metals, as by their fall (this will be explained in the answer to the next question); the heat therefore cannot be developed till the motion is stopped; and this can scarcely be said to happen till the falling body has entered the denser atmosphere beneath the flame, or, in the case of large meteorolites, even the body of the sun. The absorption of heat by evaporation must render the process comparatively slow, when the meteorolite is large.

7. How could a flame of gas produce such intense heat as comes from the sun?