155. Beginning with chemical or stuff development, we come at once to a very interesting and important question. Assuming that the atoms of the present universe were developed from the invisible, were different kinds of atoms thus developed, or were they all of one kind? To this question the chemist of last century would have replied, that undoubtedly there were many kinds of primeval atoms, and then would follow a formidable list of all these various substances which he was unable to decompose. The chemist of thirty or forty years later would still have replied to the question in the same way, but he would probably have furnished a different list of primeval elements less formidable in number. If the chemist of forty years ago had been asked, he would have furnished a list of perhaps fifty simple substances; but then, probably, the minimum would have been reached; for ask the chemist of to-day, and he will furnish a list of sixty-four so-called elements. 156. But while the number of as yet undecomposed bodies is slowly increasing by fresh discoveries, chemists are beginning to speculate as to the possibility that these so-called elements may be in reality nothing more than combinations differing in numbers and in tactical arrangement, of some one kind of primordial atoms. This idea was first entertained by Dr. Prout, the well-known physician and chemist. He pointed out that the atomic weights of the various so-called elements are very nearly all multiples of the half of that of hydrogen, so that the various elements may possibly be looked upon as formed by a grouping together of certain atoms of half the mass of the hydrogen atom. M. Stas, the distinguished Belgian chemist, instituted a laborious series of experiments with the view of testing this doctrine. He came to the conclusion that the atomic weights of the various elements were not precisely multiples of the half of that of hydrogen, there being greater differences than could possibly be accounted for by errors of experiment. His researches, however, seemed to show that in many cases there was a very near approach to Prout’s imagined law. But in no case does the discrepance appear to us greatly to exceed what may easily be attributed to unavoidable impurities in the substances operated on; say only those due to the condensation of gases in the pores of solids, which (in certain cases at least) is known to amount to a very considerable quantity. 157. From another point of view there appears to be evidence in favour of the so-called elementary bodies being built up, as more or less complex arrangements of one, or at most a few, simpler kinds of matter. There are certain groups or families amongst these elements of such a nature that the various members of one family appear to be related to each other, in the same way as the corresponding members of another family.