And if the earth was once heated to a molten state, it only required a higher heat to reduce it to a gaseous, that is to say, a nebulous condition. And, therefore, the nebular theory of the origin and formation of the earth, is not only every way reasonable, but in good degree absolutely proven. Indeed Prof. Guyot, in a paper read in September, 1874, before the Evangelical Alliance in New York, stated that it had been demonstrated in an exhaustive mathematical calculation, by Prof. Alexander, of Princeton. Whether finally settled or not, this may at present be regarded as the theory of science, the one that best explains the known facts.

The accepted theory

of Science.

One thing more we must notice before we pass, to indicate the changes that came upon this nebula in its progress toward the completion of the world.

If the earth came originally from the sun, it must be that the material in the two are the same; and it has been one of the

The earth

and sun simi

lar in compo

sition.

latest triumphs of science to prove this fact that the material of which the sun is composed, and the substance of which the earth is made, are one and the same, and may, therefore, well have come from a common laboratory.

For a long time the sun defied all attempts to analyze its substance. We could survey its sur

But

face, and note its movements, but what its composition was, was a matter of pure conjecture. the spectroscope now enables us to determine its substance, even more accurately than the telescope reveals its form and motions.

The spectroscope is an

The Spectroscope.

instrument invented some twenty years ago, by two German professors at Heidelberg.* It consists essentially of a series of prisms, and is used to determine the composition of a substance by the bright lines in its spectrum. Let us briefly explain. It has long been known that white light, as the clear light of the sun, is composed of seven colors-called the prismatic colors-so combined as to neutralize each other, producing white light. If a ray of sunlight be passed through a prism and thrown upon a screen, the colors are separated, and so separated are called the spectrum of the sun. A spectrum may also be produced by any other kind of light.

Spectrum analysis.

It has been found further, that while a heated solid or liquid substance produces a continuous spectrum-that is, one in which the colors are closely matched together —that a heated gas or vapor produces a broken spectrum-that is, consisting of bright bands or lines of light, separated by dark intervening spaces.

*Professors Bunsen and Kirchoff, 1857.

Of nearly seventy chemical elements* known to exist in nature, each produces a spectrum peculiar to itself, and therefore the composition of any substance may be determined, when reduced to a vapor, by the bright lines it yields in the spectrum, by the number of those lines, or by the order of their occurrence with reference to the dark spaces that intervene.

Now the sun produces all kinds of light. If, however, a vapor of any kind cross the path of the sun-ray-in other words, if the ray be made. to pass through a gaseous substance-a dark band will appear in the spectrum, and in that part of it the color of which is produced by the like substance. That is the substance absorbs in one condition what it produces in another.

Spectrum analysis applied

to the Sun.

The application of spectrum analysis to the sun, therefore, is on this wise. It is observed, for instance, that burning sodium, the basis of common salt, produces a yellow flame, and that in the spectrum produced by such flame, the yellow assumes the form of a broad bright band in a particular position; and that in the solar spectrum this bright band is replaced by a dark

*Prof. J. N. Lockyer, of London, on the strength of some recent experiments, ventures the suggestion that all the elements may yet be found reducible to the single element Hydrogen. The theory lacks confirmation, and seems as yet to command little or no confidence among scientific men.

band of corresponding proportions. Why is this? The matter is easily explained. Any element will absorb the kind of light it produces. If the color in the solar spectrum is absorbed, it must be by the same element that produces it. If the sodium band is absorbed-replaced by a dark band-it shows the presence of sodium in the sun.

stances.

The same rule holds good of other substances.* Thallium yields a green band; lithium a red band, with a thin orange one; hydrogen Spectra of different sub- three bands, a red, a green, and a blue one. And so on, each holding in every case its own exact position. And these all have their corresponding bands in the spectrum of the sun; whence we are led, rather driven to the conclusion that all these substances exist in the sun, as they are known to exist in the earth.

Sun and

Earth identi

stance.

It is too much, as yet, to say all the elements found in one appear in the other, for incal in sub vestigation has not gone so far; the science is comparatively new. But sufficient has been learned to warrant the presumption that the earth and sun are identical in substance, and without any reasonable doubt had a common origin.

By this method of analysis we not only learn of what material the sun consists, but have also a very certain clue as to its condition.

* See lithographic chart (frontispiece).

These elements show their colors only when heated. We must hence conclude that

Condition of

the Sun

the sun, which shows all these colors so vividly, is in a highly heated state, even if we had not sufficient evidence of that fact from the light and heat that we obtain from the sun. These considerations, some of which are of quite recent development, are regarded in the scientific world as practically settling the matter that the earth was once a nebula that came from the sun. What, then, must have been its appearance at this early date?

It is easy to understand that when all space was filled with this vaporous substance, a dense darkness must have been in and over all-darkness was on the face of the deep, or the abyss, for as yet there was no sun or moon or star, as they exist to-day; and when the change had proceeded so far that both earth and sun had assumed definite forms and motions, even approaching a solid mass at centre, that still surrounded by a deep belt of vapor, steam, or cloud, there would be no light. Impenetrable darkness would hover over all, until by processes at first unknown, the conditions should be gradually modified and the original nebula pass upon that series of changes for which it was evidently destined, and through which it is passing still. We have to do in this discourse, however,