Page:Popular Science Monthly Volume 2.djvu/335

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respects been more than doubled. Problems which were deemed insoluble, have been settled with the greatest ease. The magnitude of the discoveries already made leads us to expect still greater revelations in the future. Let us see what the spectroscope has to say for the philosophy of evolution.

Among the doctrines held by evolutionists, the all but proved Nebular Hypothesis occupies a very prominent position. Originating with Kant more than a century ago, and afterward furnished with secure foundations by Laplace, it has since striven for complete acceptance with ever-varying strength. According to this hypothesis, our solar system began existence as a nebulous cloud of incandescent vapor, which, rotating about a centre, and cooling as it revolved, cast off rings of matter that gathered into globes and became planets, while the central portion, undergoing less change, formed the sun. A vast weight of physical and mathematical evidence supported this theory, and the nebulæ seen in different parts of the heavens lent to it the confirmation of analogy. From the first, the hypothesis was strong.

But soon doubts began to arise. Larger and more powerful telescopes were constructed, and many nebulæ were resolved into clusters of stars. Astronomers began to hope that all these bodies might be similarly resolved, and the nebular hypothesis lost a little ground. But the spectroscope came apparently to the rescue. In the skilful hands of Mr. Hoggins, the narrow slit was made to receive the light of several unresolved nebulæ, and nebula after nebula gave up its secret to the observer. Some yielded spectra, consisting of from one to four bright lines, while others gave continuous bands of feeble light. The former class told the story. Spectra like theirs could belong only to the light emitted by incandescent gas, and therefore of such material, true nebulous vapor, these distant bodies consisted. But even more was revealed. The bright lines were characteristic of two well-known substances, nitrogen being the more distinct of the two, and hydrogen the less clearly visible. No other elements could be detected, nor could any good reason be found for supposing others to be present. But the main fact of the existence of genuine nebulæ was fairly demonstrated, and the nebular hypothesis received a great accession of strength. To-day it almost commands acceptance, although it is capable of being made much stronger. Even the evidence which analogy might offer in its favor is far from complete. We must look to the spectroscope for its completion.

In this connection a great variety of interesting questions suggest themselves. We assume that our planet originated from a gaseous cloud by a slow process of condensation and cooling, and point to the visible nebulæ to confirm our views. Now, in evolving a solar system from a nebula, a long series of changes would necessarily occur. We see the extremes of such a line of development, and also a few of the intermediate links. And we are at once led to ask whether we can