That all the purely physical sciences will one day be brought under a few general laws and principles, and the whole of the recognised chemical elements will be resolved into one or two material elements, is a conclusion towards which physical science seems at present slowly tending. There is certainly something fascinating in this view of the unity of nature. There is in this idea more than a purely physical interest attached to it. It has, as I hope to show in a future work, an important bearing on questions relating to the foundations of the true theory of evolution. The question of the unity of the chemical elements is one, however, yet in a hypothetical condition. Professors Liveing and Dewar, who have given attention to this subject, say: “The supposition that the different elements may be resolved into simpler constituents, or into a single one, has long been a favourite speculation with chemists; but, however probable this hypothesis may appear _à priori_, it must be acknowledged that the facts derived from the most powerful method of analytical investigation yet devised give it scant support.”[68] [Footnote 68: _Proc. Roy. Soc._ vol. xxxii. p. 230.] _Sir Benjamin Brodie on the pre-nebular condition of matter._—There are, considers Sir Benjamin Brodie, very forcible reasons which lead us to suspect that chemical substances are really composed of a primitive system of elemental bodies, analogous in their general nature to our present elements: that some of those bodies which we speak of as elements may be compounds. These ideal elements assumed by him, he says, “though now revealed to us by the numerical properties of chemical equations only as _implicit and dependent existences_, we cannot but surmise may sometimes become, or may in the past have been, _isolated and independent existences_”—as, for instance, in the case of the sun, where the temperature is excessive. “We may,” he further adds, “consider that in remote ages the temperature of matter was much higher than it is now, and that these other things [ideal elements] existed then in the state of perfect gases—separate existences—uncombined.”[69] He then refers to certain observations of Mr. Huggins and Dr. Miller on the spectra of nebulæ where one of the lines of nitrogen was found alone; and that this suggested to them that the line might have been produced by one of the elements of nitrogen; and that nitrogen may therefore be compound. He mentions as a significant fact that a large proportion of the class of elements which he has termed “composite elements” has not been found in the sun, they having probably been decomposed by the intense heat. [Footnote 69: _Ideal Chemistry_, p. 56.]