would lead to an alarming multiplication of elementary bodies, increasing our present confusion to an indefinite extent. If every possible wave-length of light represented a special element, the number of elements would be infinite. Clearly, then, this speculation, although frequently suggested, has very little to recommend it, and need not be entertained. Still, the fact of varying complexity among the elementary spectra remains to be accounted for. It certainly suggests a corresponding difference of complexity among the elements themselves, but of what nature? This question can hardly be answered directly, although it admits of interesting discussion, for which, unfortunately, we have little space to spare. Suffice it to say that spectroscopic phenomena are quite in harmony with the idea that all matter is at bottom one, our supposed atoms being really various aggregations of the same fundamental unit. It is approximately true that the simpler spectra are furnished by the elements of low atomic weight, while the multitudes of lines come from the heavier atoms. There are prominent exceptions to this rule, still it affords some support to our central idea.
But the spectroscope makes its most emphatic suggestions in favor of the unity of matter when it is applied to the study of the heavenly bodies. This subject I discussed in The Popular Science Monthly for January, 1873, and some months later Lockyer gave it prominence in England, his paper calling forth a good deal of comment. Therefore, only a brief résumé of my original suggestions is desirable now.
Everybody knows that the nebular hypothesis, as it is to-day, draws its strongest support from spectroscopic facts. There shine the nebulæ in the heavens, and the spectroscope tells us what they really are, namely, vast clouds of incandescent gas, mainly, if not entirely, hydrogen and nitrogen. If we attempt to trace the chain of evolution through which our planet is supposed to have grown, we shall find the sky is full of intermediate forms. The nebulæ themselves appear to be in various stages of development; the fixed stars or suns differ widely in chemical constitution and in temperature; our earth is most complex of all. There are no "missing links" such as the zoologist longs to discover when he tries to explain the origin of species. First, we have a nebula containing little more than hydrogen; then a very hot star with calcium, magnesium, and one or two other metals added; next comes a cooler sun in which free hydrogen is missing, but whose chemical complexity is much increased; at last we reach the true planets with their multitudes of material forms. Could there well be a more straightforward story? Could the unity of creation receive a much more ringing emphasis? We see the evolution of planets from nebulæ still going on, and parallel with it an evolution of higher from lower kinds of matter.
Just here, perhaps, is the key to the whole subject. If the elements are all in essence one, how could their many forms originate
