Such, if we mistake not, is the story epitomized—­the life history in brief—­of all the worlds of space.  They have each in its order and kind, an epoch of the beginning, then an epoch of growth and evolution, then an epoch of life—­toward which all the preceding planet history seems to tend—­and finally an epoch of death which must, in the course of infinite time, swallow from sight each planet in its turn, or at least reduce each from that condition in which it is an arena of animated existence into that state where it is a frozen and desert clod, still following its wonted path through space, still shining with a cold but cheerful face, like our moon, upon the silent abysses of the universe.

WHAT THE WORLDS ARE MADE OF.

The present century was already well advanced before there was any solid ground for the belief that the worlds of space are made of analogous or identical materials.  It was only with the invention of the spectroscope and the analysis of light that the material identity of universal nature was proved by methods which could not be doubted.  The proof came by the spectroscope.

This little instrument, though not famed as is its lordly kinsman the telescope, or even regarded with the popular favor of the microscope, has nevertheless carried us as far, and, we were about to say, taught us as much, as either of the others.  It is one thing to see the worlds afar, to note them visibly, to describe their outlines, to measure their mass and determine their motions.  It is another thing to know their constitution, the substances of which they are composed, the material condition in which they exist and the state of their progress in worldhood.  The latter work is the task of the spectroscope; and right well has it accomplished its mission.

The solar spectrum has been known from the earliest ages.  When the sun-bow was set on the background of cloud over the diluvial floods, the living beings of that age saw a spectrum—­the glorious spectrum of rain and shine.  Wherever the rays of light have been diffracted under given conditions by the agency of water drops, prism of glass or other such transparent medium, and the ray has fallen on a suitable screen, lo! there has been the beautiful spectrum of light.

The artificial, intentional production of this phenomenon of light has long been known, and both novice and scientist have tested and improved the methods of getting given results.  The child’s soap-bubble shows it in miniature splendor.  The pressure of one wet pane of glass against another reveals it.  The breakage of nearly all crystalline substances brings something of the colored effects of light; but the triangular prism of glass, suitably prepared, best of all displays the analysis of the sun-beam into the colors of which it is composed.