Just as in a burning candle the impact of the oxygen atoms in the air against the carbon and hydrogen atoms in the melted and rarefied wax or tallow produces the light and heat of the flame, so in nebula or sun the impact of the various gravitating atoms one against the other produces the light and heat by whose aid we are enabled to see and know those distant bodies.  The universe, according to this now fashionable nebular theory, began as a single vast ocean of matter of immense tenuity, spread all alike over all space as far as nowhere, and comparatively little different within itself when looked at side by side with its own final historical outcome.  In Mr. Spencer’s perspicuous phrase, evolution in this aspect is a change from the homogeneous to the heterogeneous, from the incoherent to the coherent, and from the indefinite to the definite condition.  Difficult words at first to apprehend, no doubt, and therefore to many people, as to Mr. Matthew Arnold, very repellent, but full of meaning, lucidity, and suggestiveness, if only we once take the trouble fairly and squarely to understand them.

Every sun and every star thus formed is for ever gathering in the hem of its outer robe upon itself, for ever radiating off its light and heat into surrounding space, and for ever growing denser and colder as it sets slowly towards its centre of gravity.  Our own sun and solar system may be taken as good typical working examples of how the stars thus constantly shrink into smaller and ever smaller dimensions around their own fixed centre.  Naturally, we know more about our own solar system than about any other in our own universe, and it also possesses for us a greater practical and personal interest than any outside portion of the galaxy.  Nobody can pretend to be profoundly immersed in the internal affairs of Sirius or of Alpha Centauri.  A fiery revolution in the belt of Orion would affect us less than a passing finger-ache in a certain single terrestrial baby of our own household.  Therefore I shall not apologise in any way for leaving the remainder of the sidereal universe to its unknown fate, and concentrating my attention mainly on the affairs of that solitary little, out-of-the-way, second-rate system, whereof we form an inappreciable portion.  The matter which now composes the sun and its attendant bodies (the satellites included) was once spread out, according to Laplace, to at least the furthest orbit of the outermost planet—­that is to say, so far as our present knowledge goes, the planet Neptune.  Of course, when it was expanded to that immense distance, it must have been very thin indeed, thinner than our clumsy human senses can even conceive of.  An American would say, too thin; but I put Americans out of court at once as mere irreverent scoffers.  From the orbit of Neptune, or something outside it, the faint and cloud-like mass which bore within it Caesar and his fortunes, not to mention the remainder of the earth and the solar system, began slowly to